diff --git a/internal/api/bom_handlers.go b/internal/api/bom_handlers.go
index 1e763cc..a9fdae6 100644
--- a/internal/api/bom_handlers.go
+++ b/internal/api/bom_handlers.go
@@ -1,7 +1,10 @@
 package api
 
 import (
+	"encoding/csv"
 	"encoding/json"
+	"fmt"
+	"io"
 	"net/http"
 	"strconv"
 	"strings"
@@ -14,16 +17,17 @@ import (
 
 // BOMEntryResponse represents a BOM entry in API responses.
 type BOMEntryResponse struct {
-	ID                   string   `json:"id"`
-	ChildPartNumber      string   `json:"child_part_number"`
-	ChildDescription     string   `json:"child_description"`
-	RelType              string   `json:"rel_type"`
-	Quantity             *float64 `json:"quantity"`
-	Unit                 *string  `json:"unit,omitempty"`
-	ReferenceDesignators []string `json:"reference_designators,omitempty"`
-	ChildRevision        *int     `json:"child_revision,omitempty"`
-	EffectiveRevision    int      `json:"effective_revision"`
-	Depth                *int     `json:"depth,omitempty"`
+	ID                   string         `json:"id"`
+	ChildPartNumber      string         `json:"child_part_number"`
+	ChildDescription     string         `json:"child_description"`
+	RelType              string         `json:"rel_type"`
+	Quantity             *float64       `json:"quantity"`
+	Unit                 *string        `json:"unit,omitempty"`
+	ReferenceDesignators []string       `json:"reference_designators,omitempty"`
+	ChildRevision        *int           `json:"child_revision,omitempty"`
+	EffectiveRevision    int            `json:"effective_revision"`
+	Depth                *int           `json:"depth,omitempty"`
+	Metadata             map[string]any `json:"metadata,omitempty"`
 }
 
 // WhereUsedResponse represents a where-used entry in API responses.
@@ -264,6 +268,7 @@ func (s *Server) HandleAddBOMEntry(w http.ResponseWriter, r *http.Request) {
 		ReferenceDesignators: req.ReferenceDesignators,
 		ChildRevision:        req.ChildRevision,
 		EffectiveRevision:    child.CurrentRevision,
+		Metadata:             req.Metadata,
 	}
 	if req.ChildRevision != nil {
 		entry.EffectiveRevision = *req.ChildRevision
@@ -419,6 +424,7 @@ func bomEntryToResponse(e *db.BOMEntry) BOMEntryResponse {
 		ReferenceDesignators: refDes,
 		ChildRevision:        e.ChildRevision,
 		EffectiveRevision:    e.EffectiveRevision,
+		Metadata:             e.Metadata,
 	}
 }
 
@@ -438,3 +444,380 @@ func whereUsedToResponse(e *db.BOMEntry) WhereUsedResponse {
 		ReferenceDesignators: refDes,
 	}
 }
+
+// BOM CSV headers matching the user-specified format.
+var bomCSVHeaders = []string{
+	"Item", "Level", "Source", "PN", "Seller Description",
+	"Unit Cost", "QTY", "Ext Cost", "Sourcing Link",
+}
+
+// getMetaString extracts a string value from metadata.
+func getMetaString(m map[string]any, key string) string {
+	if m == nil {
+		return ""
+	}
+	if v, ok := m[key]; ok {
+		if s, ok := v.(string); ok {
+			return s
+		}
+	}
+	return ""
+}
+
+// getMetaFloat extracts a float64 value from metadata.
+func getMetaFloat(m map[string]any, key string) (float64, bool) {
+	if m == nil {
+		return 0, false
+	}
+	v, ok := m[key]
+	if !ok {
+		return 0, false
+	}
+	switch n := v.(type) {
+	case float64:
+		return n, true
+	case json.Number:
+		f, err := n.Float64()
+		return f, err == nil
+	}
+	return 0, false
+}
+
+// HandleExportBOMCSV exports the expanded BOM as a CSV file.
+func (s *Server) HandleExportBOMCSV(w http.ResponseWriter, r *http.Request) {
+	ctx := r.Context()
+	partNumber := chi.URLParam(r, "partNumber")
+
+	item, err := s.items.GetByPartNumber(ctx, partNumber)
+	if err != nil {
+		s.logger.Error().Err(err).Msg("failed to get item")
+		writeError(w, http.StatusInternalServerError, "internal_error", "Failed to get item")
+		return
+	}
+	if item == nil {
+		writeError(w, http.StatusNotFound, "not_found", "Item not found")
+		return
+	}
+
+	entries, err := s.relationships.GetExpandedBOM(ctx, item.ID, 10)
+	if err != nil {
+		s.logger.Error().Err(err).Msg("failed to get expanded BOM")
+		writeError(w, http.StatusInternalServerError, "internal_error", "Failed to get BOM")
+		return
+	}
+
+	w.Header().Set("Content-Type", "text/csv")
+	w.Header().Set("Content-Disposition", fmt.Sprintf(`attachment; filename="%s-bom.csv"`, partNumber))
+
+	writer := csv.NewWriter(w)
+	defer writer.Flush()
+
+	// Write header
+	if err := writer.Write(bomCSVHeaders); err != nil {
+		s.logger.Error().Err(err).Msg("failed to write CSV header")
+		return
+	}
+
+	// Write rows
+	for i, e := range entries {
+		unitCost, hasUnitCost := getMetaFloat(e.Metadata, "unit_cost")
+		qty := 0.0
+		if e.Quantity != nil {
+			qty = *e.Quantity
+		}
+
+		extCost := ""
+		if hasUnitCost && qty > 0 {
+			extCost = fmt.Sprintf("%.2f", unitCost*qty)
+		}
+
+		unitCostStr := ""
+		if hasUnitCost {
+			unitCostStr = fmt.Sprintf("%.2f", unitCost)
+		}
+
+		qtyStr := ""
+		if e.Quantity != nil {
+			qtyStr = strconv.FormatFloat(*e.Quantity, 'f', -1, 64)
+		}
+
+		row := []string{
+			strconv.Itoa(i + 1),                             // Item
+			strconv.Itoa(e.Depth),                           // Level
+			getMetaString(e.Metadata, "source"),             // Source
+			e.ChildPartNumber,                               // PN
+			getMetaString(e.Metadata, "seller_description"), // Seller Description
+			unitCostStr,                                     // Unit Cost
+			qtyStr,                                          // QTY
+			extCost,                                         // Ext Cost
+			getMetaString(e.Metadata, "sourcing_link"), // Sourcing Link
+		}
+		if err := writer.Write(row); err != nil {
+			s.logger.Error().Err(err).Msg("failed to write CSV row")
+			return
+		}
+	}
+}
+
+// HandleImportBOMCSV imports BOM entries from a CSV file.
+func (s *Server) HandleImportBOMCSV(w http.ResponseWriter, r *http.Request) {
+	ctx := r.Context()
+	partNumber := chi.URLParam(r, "partNumber")
+
+	parent, err := s.items.GetByPartNumber(ctx, partNumber)
+	if err != nil {
+		s.logger.Error().Err(err).Msg("failed to get parent item")
+		writeError(w, http.StatusInternalServerError, "internal_error", "Failed to get parent item")
+		return
+	}
+	if parent == nil {
+		writeError(w, http.StatusNotFound, "not_found", "Parent item not found")
+		return
+	}
+
+	// Parse multipart form (32MB max)
+	if err := r.ParseMultipartForm(32 << 20); err != nil {
+		writeError(w, http.StatusBadRequest, "invalid_form", "Failed to parse multipart form")
+		return
+	}
+
+	file, _, err := r.FormFile("file")
+	if err != nil {
+		writeError(w, http.StatusBadRequest, "missing_file", "CSV file is required")
+		return
+	}
+	defer file.Close()
+
+	dryRun := r.FormValue("dry_run") == "true"
+	clearExisting := r.FormValue("clear_existing") == "true"
+
+	// Read CSV
+	reader := csv.NewReader(file)
+	reader.TrimLeadingSpace = true
+
+	headers, err := reader.Read()
+	if err != nil {
+		writeError(w, http.StatusBadRequest, "invalid_csv", "Failed to read CSV headers")
+		return
+	}
+
+	// Build case-insensitive header index
+	headerIdx := make(map[string]int)
+	for i, h := range headers {
+		headerIdx[strings.ToLower(strings.TrimSpace(h))] = i
+	}
+
+	// Require PN column
+	pnIdx, hasPn := headerIdx["pn"]
+	if !hasPn {
+		writeError(w, http.StatusBadRequest, "missing_column", "CSV must have a 'PN' column")
+		return
+	}
+
+	// Clear existing BOM if requested (only on real import)
+	if clearExisting && !dryRun {
+		existing, err := s.relationships.GetBOM(ctx, parent.ID)
+		if err != nil {
+			s.logger.Error().Err(err).Msg("failed to get existing BOM for clearing")
+			writeError(w, http.StatusInternalServerError, "internal_error", "Failed to clear existing BOM")
+			return
+		}
+		for _, e := range existing {
+			if err := s.relationships.Delete(ctx, e.RelationshipID); err != nil {
+				s.logger.Error().Err(err).Str("id", e.RelationshipID).Msg("failed to delete BOM entry during clear")
+			}
+		}
+	}
+
+	result := CSVImportResult{}
+	var createdItems []string
+
+	for {
+		record, err := reader.Read()
+		if err == io.EOF {
+			break
+		}
+		if err != nil {
+			result.TotalRows++
+			result.ErrorCount++
+			result.Errors = append(result.Errors, CSVImportErr{
+				Row:     result.TotalRows + 1, // +1 for header
+				Message: fmt.Sprintf("Failed to read row: %s", err.Error()),
+			})
+			continue
+		}
+
+		result.TotalRows++
+		rowNum := result.TotalRows + 1 // +1 for header
+
+		// Get part number
+		if pnIdx >= len(record) {
+			result.ErrorCount++
+			result.Errors = append(result.Errors, CSVImportErr{
+				Row:     rowNum,
+				Field:   "PN",
+				Message: "Row has fewer columns than expected",
+			})
+			continue
+		}
+
+		childPN := strings.TrimSpace(record[pnIdx])
+		if childPN == "" {
+			// Skip blank PN rows silently
+			result.TotalRows--
+			continue
+		}
+
+		// Look up child item
+		child, err := s.items.GetByPartNumber(ctx, childPN)
+		if err != nil {
+			result.ErrorCount++
+			result.Errors = append(result.Errors, CSVImportErr{
+				Row:     rowNum,
+				Field:   "PN",
+				Message: fmt.Sprintf("Error looking up item: %s", err.Error()),
+			})
+			continue
+		}
+		if child == nil {
+			result.ErrorCount++
+			result.Errors = append(result.Errors, CSVImportErr{
+				Row:     rowNum,
+				Field:   "PN",
+				Message: fmt.Sprintf("Item '%s' not found", childPN),
+			})
+			continue
+		}
+
+		// Parse quantity
+		var quantity *float64
+		if idx, ok := headerIdx["qty"]; ok && idx < len(record) {
+			qtyStr := strings.TrimSpace(record[idx])
+			if qtyStr != "" {
+				q, err := strconv.ParseFloat(qtyStr, 64)
+				if err != nil {
+					result.ErrorCount++
+					result.Errors = append(result.Errors, CSVImportErr{
+						Row:     rowNum,
+						Field:   "QTY",
+						Message: fmt.Sprintf("Invalid quantity '%s'", qtyStr),
+					})
+					continue
+				}
+				quantity = &q
+			}
+		}
+
+		// Build metadata from CSV columns
+		metadata := make(map[string]any)
+		if idx, ok := headerIdx["source"]; ok && idx < len(record) {
+			if v := strings.TrimSpace(record[idx]); v != "" {
+				metadata["source"] = v
+			}
+		}
+		if idx, ok := headerIdx["seller description"]; ok && idx < len(record) {
+			if v := strings.TrimSpace(record[idx]); v != "" {
+				metadata["seller_description"] = v
+			}
+		}
+		if idx, ok := headerIdx["unit cost"]; ok && idx < len(record) {
+			if v := strings.TrimSpace(record[idx]); v != "" {
+				// Strip leading $ or currency symbols
+				v = strings.TrimLeft(v, "$£€ ")
+				if f, err := strconv.ParseFloat(v, 64); err == nil {
+					metadata["unit_cost"] = f
+				}
+			}
+		}
+		if idx, ok := headerIdx["sourcing link"]; ok && idx < len(record) {
+			if v := strings.TrimSpace(record[idx]); v != "" {
+				metadata["sourcing_link"] = v
+			}
+		}
+
+		if len(metadata) == 0 {
+			metadata = nil
+		}
+
+		// Cycle check
+		hasCycle, err := s.relationships.HasCycle(ctx, parent.ID, child.ID)
+		if err != nil {
+			result.ErrorCount++
+			result.Errors = append(result.Errors, CSVImportErr{
+				Row:     rowNum,
+				Field:   "PN",
+				Message: fmt.Sprintf("Error checking for cycles: %s", err.Error()),
+			})
+			continue
+		}
+		if hasCycle {
+			result.ErrorCount++
+			result.Errors = append(result.Errors, CSVImportErr{
+				Row:     rowNum,
+				Field:   "PN",
+				Message: fmt.Sprintf("Adding '%s' would create a cycle", childPN),
+			})
+			continue
+		}
+
+		if dryRun {
+			result.SuccessCount++
+			continue
+		}
+
+		// Check if relationship already exists (upsert)
+		existing, err := s.relationships.GetByParentAndChild(ctx, parent.ID, child.ID)
+		if err != nil {
+			result.ErrorCount++
+			result.Errors = append(result.Errors, CSVImportErr{
+				Row:     rowNum,
+				Message: fmt.Sprintf("Error checking existing relationship: %s", err.Error()),
+			})
+			continue
+		}
+
+		if existing != nil {
+			// Update existing
+			if err := s.relationships.Update(ctx, existing.ID, nil, quantity, nil, nil, nil, metadata); err != nil {
+				result.ErrorCount++
+				result.Errors = append(result.Errors, CSVImportErr{
+					Row:     rowNum,
+					Message: fmt.Sprintf("Failed to update: %s", err.Error()),
+				})
+				continue
+			}
+		} else {
+			// Create new
+			rel := &db.Relationship{
+				ParentItemID: parent.ID,
+				ChildItemID:  child.ID,
+				RelType:      "component",
+				Quantity:     quantity,
+				Metadata:     metadata,
+			}
+			if err := s.relationships.Create(ctx, rel); err != nil {
+				result.ErrorCount++
+				result.Errors = append(result.Errors, CSVImportErr{
+					Row:     rowNum,
+					Message: fmt.Sprintf("Failed to create: %s", err.Error()),
+				})
+				continue
+			}
+			createdItems = append(createdItems, childPN)
+		}
+
+		result.SuccessCount++
+	}
+
+	result.CreatedItems = createdItems
+
+	s.logger.Info().
+		Str("parent", partNumber).
+		Bool("dry_run", dryRun).
+		Int("total", result.TotalRows).
+		Int("success", result.SuccessCount).
+		Int("errors", result.ErrorCount).
+		Msg("BOM CSV import completed")
+
+	writeJSON(w, http.StatusOK, result)
+}
diff --git a/internal/api/routes.go b/internal/api/routes.go
index 63db163..7b482e5 100644
--- a/internal/api/routes.go
+++ b/internal/api/routes.go
@@ -105,6 +105,8 @@ func NewRouter(server *Server, logger zerolog.Logger) http.Handler {
 				r.Post("/bom", server.HandleAddBOMEntry)
 				r.Get("/bom/expanded", server.HandleGetExpandedBOM)
 				r.Get("/bom/where-used", server.HandleGetWhereUsed)
+				r.Get("/bom/export.csv", server.HandleExportBOMCSV)
+				r.Post("/bom/import", server.HandleImportBOMCSV)
 				r.Put("/bom/{childPartNumber}", server.HandleUpdateBOMEntry)
 				r.Delete("/bom/{childPartNumber}", server.HandleDeleteBOMEntry)
 			})
diff --git a/internal/api/templates/items.html b/internal/api/templates/items.html
index 603eee2..218a1a3 100644
--- a/internal/api/templates/items.html
+++ b/internal/api/templates/items.html
@@ -345,6 +345,20 @@
             >
                 Revisions
             </button>
+            <button
+                class="tab-btn"
+                data-tab="bom"
+                onclick="switchDetailTab('bom')"
+            >
+                BOM
+            </button>
+            <button
+                class="tab-btn"
+                data-tab="where-used"
+                onclick="switchDetailTab('where-used')"
+            >
+                Where Used
+            </button>
         </div>
 
         <!-- Tab Content -->
@@ -355,6 +369,12 @@
             style="display: none"
         ></div>
         <div class="tab-content" id="tab-revisions" style="display: none"></div>
+        <div class="tab-content" id="tab-bom" style="display: none"></div>
+        <div
+            class="tab-content"
+            id="tab-where-used"
+            style="display: none"
+        ></div>
     </div>
 </div>
 
@@ -855,6 +875,127 @@
         min-height: 200px;
     }
 
+    /* BOM Tab */
+    .bom-toolbar {
+        display: flex;
+        justify-content: space-between;
+        align-items: center;
+        margin-bottom: 1rem;
+        gap: 0.5rem;
+        flex-wrap: wrap;
+    }
+    .bom-toolbar-actions {
+        display: flex;
+        gap: 0.5rem;
+    }
+    .bom-toolbar .btn {
+        padding: 0.4rem 0.75rem;
+        font-size: 0.8rem;
+    }
+    .bom-table {
+        width: 100%;
+        border-collapse: collapse;
+        font-size: 0.85rem;
+    }
+    .bom-table th {
+        padding: 0.5rem 0.75rem;
+        background: var(--ctp-base);
+        color: var(--ctp-subtext1);
+        font-weight: 600;
+        font-size: 0.75rem;
+        text-transform: uppercase;
+        letter-spacing: 0.05em;
+        text-align: left;
+        border-bottom: 1px solid var(--ctp-surface1);
+    }
+    .bom-table td {
+        padding: 0.5rem 0.75rem;
+        border-bottom: 1px solid var(--ctp-surface1);
+        color: var(--ctp-text);
+    }
+    .bom-table tr:hover {
+        background: var(--ctp-surface1);
+    }
+    .bom-table tr:last-child td {
+        border-bottom: none;
+    }
+    .bom-table .pn-link {
+        cursor: pointer;
+        color: var(--ctp-peach);
+        font-family: "JetBrains Mono", "Fira Code", monospace;
+        font-weight: 500;
+    }
+    .bom-table .pn-link:hover {
+        text-decoration: underline;
+    }
+    .bom-cost {
+        font-family: "JetBrains Mono", "Fira Code", monospace;
+        text-align: right;
+    }
+    .bom-summary {
+        display: flex;
+        justify-content: flex-end;
+        padding: 0.75rem;
+        font-weight: 600;
+        border-top: 2px solid var(--ctp-surface2);
+        font-family: "JetBrains Mono", "Fira Code", monospace;
+        color: var(--ctp-green);
+    }
+    .bom-empty {
+        text-align: center;
+        padding: 2rem;
+        color: var(--ctp-subtext0);
+    }
+    .bom-add-form {
+        display: grid;
+        grid-template-columns: 1fr 1fr;
+        gap: 0.75rem;
+        padding: 1rem;
+        background: var(--ctp-base);
+        border-radius: 0.5rem;
+        margin-bottom: 1rem;
+    }
+    .bom-add-form .form-group {
+        margin-bottom: 0;
+    }
+    .bom-add-form .full-width {
+        grid-column: 1 / -1;
+    }
+    .bom-import-area {
+        border: 2px dashed var(--ctp-surface2);
+        border-radius: 0.5rem;
+        padding: 1.5rem;
+        text-align: center;
+        margin-bottom: 1rem;
+        transition: border-color 0.2s;
+    }
+    .bom-import-area:hover {
+        border-color: var(--ctp-mauve);
+    }
+    .bom-import-results {
+        margin-top: 1rem;
+        padding: 1rem;
+        border-radius: 0.5rem;
+        background: var(--ctp-base);
+    }
+    .bom-import-results.success {
+        border-left: 3px solid var(--ctp-green);
+    }
+    .bom-import-results.error {
+        border-left: 3px solid var(--ctp-red);
+    }
+    .bom-import-results.warning {
+        border-left: 3px solid var(--ctp-yellow);
+    }
+    .bom-actions {
+        display: flex;
+        gap: 0.25rem;
+    }
+    .bom-actions .btn {
+        padding: 0.2rem 0.5rem;
+        font-size: 0.75rem;
+    }
+
     /* Properties Editor */
     .properties-editor {
         padding: 0.5rem 0;
@@ -1592,6 +1733,14 @@
             content.style.display = "none";
         });
         document.getElementById(`tab-${tab}`).style.display = "block";
+
+        // Lazy-load BOM and Where Used data
+        if (tab === "bom" && currentItemPartNumber) {
+            loadBOMTab(currentItemPartNumber);
+        }
+        if (tab === "where-used" && currentItemPartNumber) {
+            loadWhereUsedTab(currentItemPartNumber);
+        }
     }
 
     // Format file size
@@ -1618,6 +1767,8 @@
             '<div class="loading"><div class="spinner"></div></div>';
         document.getElementById("tab-properties").innerHTML = "";
         document.getElementById("tab-revisions").innerHTML = "";
+        document.getElementById("tab-bom").innerHTML = "";
+        document.getElementById("tab-where-used").innerHTML = "";
 
         try {
             const [itemRes, revsRes] = await Promise.all([
@@ -2445,6 +2596,493 @@
         }
     }
 
+    // ========================================
+    // BOM Tab Functions
+    // ========================================
+
+    let bomData = [];
+
+    function escapeAttr(str) {
+        return String(str)
+            .replace(/&/g, "&amp;")
+            .replace(/"/g, "&quot;")
+            .replace(/'/g, "&#39;")
+            .replace(/</g, "&lt;")
+            .replace(/>/g, "&gt;");
+    }
+
+    async function loadBOMTab(partNumber) {
+        const container = document.getElementById("tab-bom");
+        container.innerHTML =
+            '<div class="loading"><div class="spinner"></div></div>';
+
+        try {
+            const response = await fetch(`/api/items/${partNumber}/bom`);
+            if (!response.ok) throw new Error("Failed to load BOM");
+            bomData = await response.json();
+            renderBOMTab(partNumber, bomData);
+        } catch (error) {
+            container.innerHTML = `<p style="color: var(--ctp-red);">Error: ${error.message}</p>`;
+        }
+    }
+
+    function renderBOMTab(partNumber, entries) {
+        const container = document.getElementById("tab-bom");
+        const escapedPN = escapeAttr(partNumber);
+
+        let totalExtCost = 0;
+        entries.forEach((e) => {
+            const unitCost = e.metadata?.unit_cost || 0;
+            const qty = e.quantity || 0;
+            totalExtCost += unitCost * qty;
+        });
+
+        let html = `
+            <div class="bom-toolbar">
+                <span style="color: var(--ctp-subtext0); font-size: 0.85rem;">
+                    ${entries.length} component${entries.length !== 1 ? "s" : ""}
+                </span>
+                <div class="bom-toolbar-actions">
+                    <button class="btn btn-secondary" onclick="exportBOMCSV('${escapedPN}')">Export CSV</button>
+                    <button class="btn btn-secondary" onclick="showBOMImport('${escapedPN}')">Import CSV</button>
+                    <button class="btn btn-primary" onclick="showAddBOMForm('${escapedPN}')">+ Add</button>
+                </div>
+            </div>
+            <div id="bom-add-container"></div>
+            <div id="bom-import-container"></div>`;
+
+        if (entries.length === 0) {
+            html +=
+                '<div class="bom-empty"><p>No BOM entries.</p><p style="font-size:0.85rem;">Add components or import a CSV.</p></div>';
+        } else {
+            html += `
+            <div class="table-container">
+                <table class="bom-table">
+                    <thead>
+                        <tr>
+                            <th>#</th>
+                            <th>PN</th>
+                            <th>Source</th>
+                            <th>Seller Description</th>
+                            <th style="text-align:right">Unit Cost</th>
+                            <th style="text-align:right">QTY</th>
+                            <th style="text-align:right">Ext Cost</th>
+                            <th>Link</th>
+                            <th style="width:90px">Actions</th>
+                        </tr>
+                    </thead>
+                    <tbody>`;
+
+            entries.forEach((e, idx) => {
+                const unitCost = e.metadata?.unit_cost || 0;
+                const qty = e.quantity || 0;
+                const extCost = unitCost * qty;
+                const source = escapeHtml(e.metadata?.source || "");
+                const sellerDesc = escapeHtml(
+                    e.metadata?.seller_description || e.child_description || "",
+                );
+                const sourcingLink = e.metadata?.sourcing_link || "";
+                const childPN = escapeAttr(e.child_part_number);
+
+                html += `
+                    <tr>
+                        <td>${idx + 1}</td>
+                        <td><span class="pn-link" onclick="showItemDetail('${childPN}')">${escapeHtml(e.child_part_number)}</span></td>
+                        <td>${source}</td>
+                        <td>${sellerDesc}</td>
+                        <td class="bom-cost">${unitCost ? "$" + unitCost.toFixed(2) : ""}</td>
+                        <td class="bom-cost">${qty || ""}</td>
+                        <td class="bom-cost">${extCost ? "$" + extCost.toFixed(2) : ""}</td>
+                        <td>${sourcingLink ? `<a href="${escapeAttr(sourcingLink)}" target="_blank" rel="noopener" style="color:var(--ctp-blue);font-size:0.8rem;">Link</a>` : ""}</td>
+                        <td>
+                            <div class="bom-actions">
+                                <button class="btn btn-secondary" onclick="editBOMEntry('${escapedPN}', ${idx})">Edit</button>
+                                <button class="btn btn-secondary" style="color:var(--ctp-red)" onclick="deleteBOMEntry('${escapedPN}', '${childPN}')">Del</button>
+                            </div>
+                        </td>
+                    </tr>`;
+            });
+
+            html += `</tbody></table></div>`;
+            if (totalExtCost > 0) {
+                html += `<div class="bom-summary">Total: $${totalExtCost.toFixed(2)}</div>`;
+            }
+        }
+
+        container.innerHTML = html;
+    }
+
+    function showAddBOMForm(partNumber) {
+        const container = document.getElementById("bom-add-container");
+        document.getElementById("bom-import-container").innerHTML = "";
+        const escapedPN = escapeAttr(partNumber);
+        container.innerHTML = `
+            <div class="bom-add-form">
+                <div class="form-group">
+                    <label class="form-label">Part Number *</label>
+                    <input type="text" class="form-input" id="bom-add-pn" placeholder="e.g. F01-0001">
+                </div>
+                <div class="form-group">
+                    <label class="form-label">Quantity</label>
+                    <input type="number" class="form-input" id="bom-add-qty" step="any" value="1">
+                </div>
+                <div class="form-group">
+                    <label class="form-label">Source</label>
+                    <input type="text" class="form-input" id="bom-add-source" placeholder="e.g. DigiKey">
+                </div>
+                <div class="form-group">
+                    <label class="form-label">Seller Description</label>
+                    <input type="text" class="form-input" id="bom-add-seller-desc">
+                </div>
+                <div class="form-group">
+                    <label class="form-label">Unit Cost</label>
+                    <input type="number" class="form-input" id="bom-add-unit-cost" step="0.01" placeholder="0.00">
+                </div>
+                <div class="form-group">
+                    <label class="form-label">Sourcing Link</label>
+                    <input type="url" class="form-input" id="bom-add-sourcing-link" placeholder="https://...">
+                </div>
+                <div class="form-group full-width" style="display:flex;gap:0.5rem;justify-content:flex-end;">
+                    <button class="btn btn-secondary" onclick="document.getElementById('bom-add-container').innerHTML=''">Cancel</button>
+                    <button class="btn btn-primary" onclick="submitAddBOM('${escapedPN}')">Add</button>
+                </div>
+            </div>`;
+        document.getElementById("bom-add-pn").focus();
+    }
+
+    async function submitAddBOM(partNumber) {
+        const pn = document.getElementById("bom-add-pn").value.trim();
+        if (!pn) {
+            alert("Part number is required");
+            return;
+        }
+
+        const qty =
+            parseFloat(document.getElementById("bom-add-qty").value) || null;
+        const metadata = {};
+        const source = document.getElementById("bom-add-source").value.trim();
+        const sellerDesc = document
+            .getElementById("bom-add-seller-desc")
+            .value.trim();
+        const unitCost = parseFloat(
+            document.getElementById("bom-add-unit-cost").value,
+        );
+        const sourcingLink = document
+            .getElementById("bom-add-sourcing-link")
+            .value.trim();
+
+        if (source) metadata.source = source;
+        if (sellerDesc) metadata.seller_description = sellerDesc;
+        if (!isNaN(unitCost)) metadata.unit_cost = unitCost;
+        if (sourcingLink) metadata.sourcing_link = sourcingLink;
+
+        try {
+            const response = await fetch(`/api/items/${partNumber}/bom`, {
+                method: "POST",
+                headers: { "Content-Type": "application/json" },
+                body: JSON.stringify({
+                    child_part_number: pn,
+                    rel_type: "component",
+                    quantity: qty,
+                    metadata:
+                        Object.keys(metadata).length > 0 ? metadata : undefined,
+                }),
+            });
+            if (!response.ok) {
+                const err = await response.json();
+                alert(err.message || err.error);
+                return;
+            }
+            document.getElementById("bom-add-container").innerHTML = "";
+            loadBOMTab(partNumber);
+        } catch (error) {
+            alert(error.message);
+        }
+    }
+
+    function editBOMEntry(partNumber, idx) {
+        const e = bomData[idx];
+        if (!e) return;
+        const container = document.getElementById("bom-add-container");
+        document.getElementById("bom-import-container").innerHTML = "";
+        const escapedPN = escapeAttr(partNumber);
+        const childPN = escapeAttr(e.child_part_number);
+
+        container.innerHTML = `
+            <div class="bom-add-form">
+                <div class="form-group">
+                    <label class="form-label">Part Number</label>
+                    <input type="text" class="form-input" value="${childPN}" disabled>
+                </div>
+                <div class="form-group">
+                    <label class="form-label">Quantity</label>
+                    <input type="number" class="form-input" id="bom-edit-qty" step="any" value="${e.quantity || ""}">
+                </div>
+                <div class="form-group">
+                    <label class="form-label">Source</label>
+                    <input type="text" class="form-input" id="bom-edit-source" value="${escapeAttr(e.metadata?.source || "")}">
+                </div>
+                <div class="form-group">
+                    <label class="form-label">Seller Description</label>
+                    <input type="text" class="form-input" id="bom-edit-seller-desc" value="${escapeAttr(e.metadata?.seller_description || "")}">
+                </div>
+                <div class="form-group">
+                    <label class="form-label">Unit Cost</label>
+                    <input type="number" class="form-input" id="bom-edit-unit-cost" step="0.01" value="${e.metadata?.unit_cost || ""}">
+                </div>
+                <div class="form-group">
+                    <label class="form-label">Sourcing Link</label>
+                    <input type="url" class="form-input" id="bom-edit-sourcing-link" value="${escapeAttr(e.metadata?.sourcing_link || "")}">
+                </div>
+                <div class="form-group full-width" style="display:flex;gap:0.5rem;justify-content:flex-end;">
+                    <button class="btn btn-secondary" onclick="document.getElementById('bom-add-container').innerHTML=''">Cancel</button>
+                    <button class="btn btn-primary" onclick="submitEditBOM('${escapedPN}', '${childPN}')">Save</button>
+                </div>
+            </div>`;
+    }
+
+    async function submitEditBOM(partNumber, childPN) {
+        const qty = parseFloat(document.getElementById("bom-edit-qty").value);
+        const metadata = {};
+        const source = document.getElementById("bom-edit-source").value.trim();
+        const sellerDesc = document
+            .getElementById("bom-edit-seller-desc")
+            .value.trim();
+        const unitCost = parseFloat(
+            document.getElementById("bom-edit-unit-cost").value,
+        );
+        const sourcingLink = document
+            .getElementById("bom-edit-sourcing-link")
+            .value.trim();
+
+        if (source) metadata.source = source;
+        if (sellerDesc) metadata.seller_description = sellerDesc;
+        if (!isNaN(unitCost)) metadata.unit_cost = unitCost;
+        if (sourcingLink) metadata.sourcing_link = sourcingLink;
+
+        const body = {};
+        if (!isNaN(qty)) body.quantity = qty;
+        if (Object.keys(metadata).length > 0) body.metadata = metadata;
+
+        try {
+            const response = await fetch(
+                `/api/items/${partNumber}/bom/${childPN}`,
+                {
+                    method: "PUT",
+                    headers: { "Content-Type": "application/json" },
+                    body: JSON.stringify(body),
+                },
+            );
+            if (!response.ok) {
+                const err = await response.json();
+                alert(err.message || err.error);
+                return;
+            }
+            document.getElementById("bom-add-container").innerHTML = "";
+            loadBOMTab(partNumber);
+        } catch (error) {
+            alert(error.message);
+        }
+    }
+
+    async function deleteBOMEntry(partNumber, childPN) {
+        if (!confirm(`Remove ${childPN} from BOM?`)) return;
+        try {
+            const response = await fetch(
+                `/api/items/${partNumber}/bom/${childPN}`,
+                { method: "DELETE" },
+            );
+            if (!response.ok && response.status !== 204) {
+                const err = await response.json();
+                alert(err.message || err.error);
+                return;
+            }
+            loadBOMTab(partNumber);
+        } catch (error) {
+            alert(error.message);
+        }
+    }
+
+    function exportBOMCSV(partNumber) {
+        window.location.href = `/api/items/${partNumber}/bom/export.csv`;
+    }
+
+    function showBOMImport(partNumber) {
+        const container = document.getElementById("bom-import-container");
+        document.getElementById("bom-add-container").innerHTML = "";
+        const escapedPN = escapeAttr(partNumber);
+        container.innerHTML = `
+            <div class="bom-import-area">
+                <div style="margin-bottom:0.75rem;">
+                    <input type="file" id="bom-import-file" accept=".csv,text/csv" style="display:none"
+                        onchange="document.getElementById('bom-file-label').textContent = this.files[0]?.name || 'Choose a CSV file'">
+                    <button class="btn btn-secondary" onclick="document.getElementById('bom-import-file').click()">Choose File</button>
+                    <span id="bom-file-label" style="margin-left:0.5rem;color:var(--ctp-subtext0);font-size:0.85rem;">No file selected</span>
+                </div>
+                <div style="display:flex;gap:1rem;justify-content:center;align-items:center;margin-bottom:0.75rem;">
+                    <label style="display:flex;align-items:center;gap:0.4rem;color:var(--ctp-subtext1);font-size:0.85rem;">
+                        <input type="checkbox" id="bom-import-dry-run" checked> Dry run (validate only)
+                    </label>
+                    <label style="display:flex;align-items:center;gap:0.4rem;color:var(--ctp-subtext1);font-size:0.85rem;">
+                        <input type="checkbox" id="bom-import-clear"> Replace existing BOM
+                    </label>
+                </div>
+                <div id="bom-import-results"></div>
+                <div style="display:flex;gap:0.5rem;justify-content:center;margin-top:0.75rem;">
+                    <button class="btn btn-secondary" onclick="document.getElementById('bom-import-container').innerHTML=''">Cancel</button>
+                    <button class="btn btn-primary" id="bom-import-btn" onclick="submitBOMImport('${escapedPN}')">Validate</button>
+                </div>
+            </div>`;
+    }
+
+    async function submitBOMImport(partNumber) {
+        const fileInput = document.getElementById("bom-import-file");
+        if (!fileInput.files || !fileInput.files[0]) {
+            alert("Select a CSV file");
+            return;
+        }
+
+        const dryRun = document.getElementById("bom-import-dry-run").checked;
+        const clearExisting =
+            document.getElementById("bom-import-clear").checked;
+        const btn = document.getElementById("bom-import-btn");
+        const resultsDiv = document.getElementById("bom-import-results");
+
+        const formData = new FormData();
+        formData.append("file", fileInput.files[0]);
+        formData.append("dry_run", dryRun.toString());
+        formData.append("clear_existing", clearExisting.toString());
+
+        btn.textContent = dryRun ? "Validating..." : "Importing...";
+        btn.disabled = true;
+
+        try {
+            const response = await fetch(
+                `/api/items/${partNumber}/bom/import`,
+                {
+                    method: "POST",
+                    body: formData,
+                },
+            );
+
+            const contentType = response.headers.get("content-type");
+            if (!contentType || !contentType.includes("application/json")) {
+                const text = await response.text();
+                resultsDiv.innerHTML = `<div class="bom-import-results error"><strong>Error</strong><p>Server returned non-JSON response (status ${response.status}).</p></div>`;
+                return;
+            }
+
+            const result = await response.json();
+
+            if (!response.ok) {
+                resultsDiv.innerHTML = `<div class="bom-import-results error"><strong>Error</strong><p>${result.message || result.error}</p></div>`;
+                return;
+            }
+
+            let statusClass = "success";
+            if (result.error_count > 0 && result.success_count > 0)
+                statusClass = "warning";
+            else if (result.error_count > 0) statusClass = "error";
+
+            let rhtml = `<div class="bom-import-results ${statusClass}">`;
+            rhtml += `<strong>${dryRun ? "Validation" : "Import"} Complete</strong>`;
+            rhtml += `<div style="display:flex;gap:1.5rem;margin:0.5rem 0;">`;
+            rhtml += `<span>Total: ${result.total_rows}</span>`;
+            rhtml += `<span style="color:var(--ctp-green)">${dryRun ? "Valid" : "Success"}: ${result.success_count}</span>`;
+            rhtml += `<span style="color:var(--ctp-red)">Errors: ${result.error_count}</span>`;
+            rhtml += `</div>`;
+
+            if (result.errors && result.errors.length > 0) {
+                rhtml += `<div style="margin-top:0.5rem;font-size:0.85rem;">`;
+                result.errors.slice(0, 20).forEach((err) => {
+                    rhtml += `<div style="color:var(--ctp-red);margin:0.2rem 0;">Row ${err.row}: ${err.field ? `[${err.field}] ` : ""}${escapeHtml(err.message)}</div>`;
+                });
+                if (result.errors.length > 20) {
+                    rhtml += `<div style="color:var(--ctp-subtext0);">...and ${result.errors.length - 20} more errors</div>`;
+                }
+                rhtml += `</div>`;
+            }
+
+            if (result.created_items && result.created_items.length > 0) {
+                rhtml += `<div style="margin-top:0.5rem;font-size:0.85rem;"><strong>Added:</strong> ${result.created_items.map(escapeHtml).join(", ")}</div>`;
+            }
+
+            rhtml += `</div>`;
+            resultsDiv.innerHTML = rhtml;
+
+            if (
+                dryRun &&
+                result.success_count > 0 &&
+                result.error_count === 0
+            ) {
+                btn.textContent = "Import Now";
+                document.getElementById("bom-import-dry-run").checked = false;
+            } else if (!dryRun && result.success_count > 0) {
+                loadBOMTab(partNumber);
+                btn.textContent = "Done";
+            } else {
+                btn.textContent = "Validate";
+            }
+        } catch (error) {
+            resultsDiv.innerHTML = `<div class="bom-import-results error"><strong>Error</strong><p>${error.message}</p></div>`;
+        } finally {
+            btn.disabled = false;
+        }
+    }
+
+    // ========================================
+    // Where Used Tab
+    // ========================================
+
+    async function loadWhereUsedTab(partNumber) {
+        const container = document.getElementById("tab-where-used");
+        container.innerHTML =
+            '<div class="loading"><div class="spinner"></div></div>';
+
+        try {
+            const response = await fetch(
+                `/api/items/${partNumber}/bom/where-used`,
+            );
+            if (!response.ok) throw new Error("Failed to load where-used data");
+            const entries = await response.json();
+
+            if (entries.length === 0) {
+                container.innerHTML =
+                    '<div class="bom-empty"><p>This item is not used in any assemblies.</p></div>';
+                return;
+            }
+
+            let html = `
+            <div class="table-container">
+                <table class="bom-table">
+                    <thead>
+                        <tr>
+                            <th>Parent PN</th>
+                            <th>Description</th>
+                            <th style="text-align:right">QTY</th>
+                            <th>Type</th>
+                        </tr>
+                    </thead>
+                    <tbody>`;
+
+            entries.forEach((e) => {
+                const parentPN = escapeAttr(e.parent_part_number);
+                html += `
+                    <tr>
+                        <td><span class="pn-link" onclick="showItemDetail('${parentPN}')">${escapeHtml(e.parent_part_number)}</span></td>
+                        <td>${escapeHtml(e.parent_description || "")}</td>
+                        <td class="bom-cost">${e.quantity || ""}</td>
+                        <td><span class="item-type item-type-part">${escapeHtml(e.rel_type)}</span></td>
+                    </tr>`;
+            });
+
+            html += `</tbody></table></div>`;
+            container.innerHTML = html;
+        } catch (error) {
+            container.innerHTML = `<p style="color: var(--ctp-red);">Error: ${error.message}</p>`;
+        }
+    }
+
     // Initialize
     loadSchema();
     loadProjectCodes();
diff --git a/internal/db/relationships.go b/internal/db/relationships.go
index dfcc0ed..5b70225 100644
--- a/internal/db/relationships.go
+++ b/internal/db/relationships.go
@@ -41,6 +41,7 @@ type BOMEntry struct {
 	ReferenceDesignators []string
 	ChildRevision        *int
 	EffectiveRevision    int
+	Metadata             map[string]any
 }
 
 // BOMTreeEntry extends BOMEntry with depth for multi-level BOM expansion.
@@ -243,7 +244,8 @@ func (r *RelationshipRepository) GetBOM(ctx context.Context, parentItemID string
 		       rel.child_item_id, child.part_number, child.description,
 		       rel.rel_type, rel.quantity, rel.unit,
 		       rel.reference_designators, rel.child_revision,
-		       COALESCE(rel.child_revision, child.current_revision) AS effective_revision
+		       COALESCE(rel.child_revision, child.current_revision) AS effective_revision,
+		       rel.metadata
 		FROM relationships rel
 		JOIN items parent ON parent.id = rel.parent_item_id
 		JOIN items child ON child.id = rel.child_item_id
@@ -267,7 +269,8 @@ func (r *RelationshipRepository) GetWhereUsed(ctx context.Context, childItemID s
 		       rel.child_item_id, child.part_number, child.description,
 		       rel.rel_type, rel.quantity, rel.unit,
 		       rel.reference_designators, rel.child_revision,
-		       COALESCE(rel.child_revision, child.current_revision) AS effective_revision
+		       COALESCE(rel.child_revision, child.current_revision) AS effective_revision,
+		       rel.metadata
 		FROM relationships rel
 		JOIN items parent ON parent.id = rel.parent_item_id
 		JOIN items child ON child.id = rel.child_item_id
@@ -301,6 +304,7 @@ func (r *RelationshipRepository) GetExpandedBOM(ctx context.Context, parentItemI
 			       rel.rel_type, rel.quantity, rel.unit,
 			       rel.reference_designators, rel.child_revision,
 			       COALESCE(rel.child_revision, child.current_revision) AS effective_revision,
+			       rel.metadata,
 			       1 AS depth
 			FROM relationships rel
 			JOIN items parent ON parent.id = rel.parent_item_id
@@ -319,6 +323,7 @@ func (r *RelationshipRepository) GetExpandedBOM(ctx context.Context, parentItemI
 			       rel.rel_type, rel.quantity, rel.unit,
 			       rel.reference_designators, rel.child_revision,
 			       COALESCE(rel.child_revision, child.current_revision),
+			       rel.metadata,
 			       bt.depth + 1
 			FROM relationships rel
 			JOIN items parent ON parent.id = rel.parent_item_id
@@ -331,7 +336,7 @@ func (r *RelationshipRepository) GetExpandedBOM(ctx context.Context, parentItemI
 		SELECT id, parent_item_id, parent_part_number, parent_description,
 		       child_item_id, child_part_number, child_description,
 		       rel_type, quantity, unit, reference_designators,
-		       child_revision, effective_revision, depth
+		       child_revision, effective_revision, metadata, depth
 		FROM bom_tree
 		ORDER BY depth, child_part_number
 	`, parentItemID, maxDepth)
@@ -344,12 +349,13 @@ func (r *RelationshipRepository) GetExpandedBOM(ctx context.Context, parentItemI
 	for rows.Next() {
 		e := &BOMTreeEntry{}
 		var parentDesc, childDesc *string
+		var metadataJSON []byte
 		err := rows.Scan(
 			&e.RelationshipID, &e.ParentItemID, &e.ParentPartNumber, &parentDesc,
 			&e.ChildItemID, &e.ChildPartNumber, &childDesc,
 			&e.RelType, &e.Quantity, &e.Unit,
 			&e.ReferenceDesignators, &e.ChildRevision,
-			&e.EffectiveRevision, &e.Depth,
+			&e.EffectiveRevision, &metadataJSON, &e.Depth,
 		)
 		if err != nil {
 			return nil, fmt.Errorf("scanning BOM tree entry: %w", err)
@@ -360,6 +366,11 @@ func (r *RelationshipRepository) GetExpandedBOM(ctx context.Context, parentItemI
 		if childDesc != nil {
 			e.ChildDescription = *childDesc
 		}
+		if metadataJSON != nil {
+			if err := json.Unmarshal(metadataJSON, &e.Metadata); err != nil {
+				return nil, fmt.Errorf("unmarshaling BOM entry metadata: %w", err)
+			}
+		}
 		entries = append(entries, e)
 	}
 
@@ -407,12 +418,14 @@ func scanBOMEntries(rows pgx.Rows) ([]*BOMEntry, error) {
 	for rows.Next() {
 		e := &BOMEntry{}
 		var parentDesc, childDesc *string
+		var metadataJSON []byte
 		err := rows.Scan(
 			&e.RelationshipID, &e.ParentItemID, &e.ParentPartNumber, &parentDesc,
 			&e.ChildItemID, &e.ChildPartNumber, &childDesc,
 			&e.RelType, &e.Quantity, &e.Unit,
 			&e.ReferenceDesignators, &e.ChildRevision,
 			&e.EffectiveRevision,
+			&metadataJSON,
 		)
 		if err != nil {
 			return nil, fmt.Errorf("scanning BOM entry: %w", err)
@@ -423,6 +436,11 @@ func scanBOMEntries(rows pgx.Rows) ([]*BOMEntry, error) {
 		if childDesc != nil {
 			e.ChildDescription = *childDesc
 		}
+		if metadataJSON != nil {
+			if err := json.Unmarshal(metadataJSON, &e.Metadata); err != nil {
+				return nil, fmt.Errorf("unmarshaling BOM entry metadata: %w", err)
+			}
+		}
 		entries = append(entries, e)
 	}