diff --git a/internal/modules/modules.go b/internal/modules/modules.go
new file mode 100644
index 0000000..c0fb3e0
--- /dev/null
+++ b/internal/modules/modules.go
@@ -0,0 +1,163 @@
+// Package modules provides the module registry for Silo.
+// Each module groups API endpoints, UI views, and configuration.
+// Modules can be required (always on) or optional (admin-toggleable).
+package modules
+
+import (
+	"fmt"
+	"sort"
+	"sync"
+)
+
+// Module IDs.
+const (
+	Core     = "core"
+	Schemas  = "schemas"
+	Storage  = "storage"
+	Auth     = "auth"
+	Projects = "projects"
+	Audit    = "audit"
+	Odoo     = "odoo"
+	FreeCAD  = "freecad"
+	Jobs     = "jobs"
+	DAG      = "dag"
+)
+
+// ModuleInfo describes a module's metadata.
+type ModuleInfo struct {
+	ID             string
+	Name           string
+	Description    string
+	Required       bool     // cannot be disabled
+	DefaultEnabled bool     // initial state for optional modules
+	DependsOn      []string // module IDs this module requires
+	Version        string
+}
+
+// registry entries with their runtime enabled state.
+type moduleState struct {
+	ModuleInfo
+	enabled bool
+}
+
+// Registry holds all module definitions and their enabled state.
+type Registry struct {
+	mu      sync.RWMutex
+	modules map[string]*moduleState
+}
+
+// builtinModules defines the complete set of Silo modules.
+var builtinModules = []ModuleInfo{
+	{ID: Core, Name: "Core PDM", Description: "Items, revisions, files, BOM, search, import/export", Required: true, Version: "0.2"},
+	{ID: Schemas, Name: "Schemas", Description: "Part numbering schema parsing and segment management", Required: true},
+	{ID: Storage, Name: "Storage", Description: "MinIO/S3 file storage, presigned uploads", Required: true},
+	{ID: Auth, Name: "Authentication", Description: "Local, LDAP, OIDC authentication and RBAC", DefaultEnabled: true},
+	{ID: Projects, Name: "Projects", Description: "Project management and item tagging", DefaultEnabled: true},
+	{ID: Audit, Name: "Audit", Description: "Audit logging, completeness scoring", DefaultEnabled: true},
+	{ID: Odoo, Name: "Odoo ERP", Description: "Odoo integration (config, sync-log, push/pull)", DependsOn: []string{Auth}},
+	{ID: FreeCAD, Name: "Create Integration", Description: "URI scheme, executable path, client settings", DefaultEnabled: true},
+	{ID: Jobs, Name: "Job Queue", Description: "Async compute jobs, runner management", DependsOn: []string{Auth}},
+	{ID: DAG, Name: "Dependency DAG", Description: "Feature DAG sync, validation states, interference detection", DependsOn: []string{Jobs}},
+}
+
+// NewRegistry creates a registry with all builtin modules set to their default state.
+func NewRegistry() *Registry {
+	r := &Registry{modules: make(map[string]*moduleState, len(builtinModules))}
+	for _, m := range builtinModules {
+		enabled := m.Required || m.DefaultEnabled
+		r.modules[m.ID] = &moduleState{ModuleInfo: m, enabled: enabled}
+	}
+	return r
+}
+
+// IsEnabled returns whether a module is currently enabled.
+func (r *Registry) IsEnabled(id string) bool {
+	r.mu.RLock()
+	defer r.mu.RUnlock()
+	if m, ok := r.modules[id]; ok {
+		return m.enabled
+	}
+	return false
+}
+
+// SetEnabled changes a module's enabled state with dependency validation.
+func (r *Registry) SetEnabled(id string, enabled bool) error {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+
+	m, ok := r.modules[id]
+	if !ok {
+		return fmt.Errorf("unknown module %q", id)
+	}
+
+	if m.Required {
+		return fmt.Errorf("module %q is required and cannot be disabled", id)
+	}
+
+	if enabled {
+		// Check that all dependencies are enabled.
+		for _, dep := range m.DependsOn {
+			if dm, ok := r.modules[dep]; ok && !dm.enabled {
+				return fmt.Errorf("cannot enable %q: dependency %q is disabled", id, dep)
+			}
+		}
+	} else {
+		// Check that no enabled module depends on this one.
+		for _, other := range r.modules {
+			if !other.enabled || other.ID == id {
+				continue
+			}
+			for _, dep := range other.DependsOn {
+				if dep == id {
+					return fmt.Errorf("cannot disable %q: module %q depends on it", id, other.ID)
+				}
+			}
+		}
+	}
+
+	m.enabled = enabled
+	return nil
+}
+
+// All returns info for every module, sorted by ID.
+func (r *Registry) All() []ModuleInfo {
+	r.mu.RLock()
+	defer r.mu.RUnlock()
+
+	out := make([]ModuleInfo, 0, len(r.modules))
+	for _, m := range r.modules {
+		out = append(out, m.ModuleInfo)
+	}
+	sort.Slice(out, func(i, j int) bool { return out[i].ID < out[j].ID })
+	return out
+}
+
+// Get returns info for a single module, or nil if not found.
+func (r *Registry) Get(id string) *ModuleInfo {
+	r.mu.RLock()
+	defer r.mu.RUnlock()
+	if m, ok := r.modules[id]; ok {
+		info := m.ModuleInfo
+		return &info
+	}
+	return nil
+}
+
+// ValidateDependencies checks that every enabled module's dependencies
+// are also enabled. Returns the first violation found.
+func (r *Registry) ValidateDependencies() error {
+	r.mu.RLock()
+	defer r.mu.RUnlock()
+
+	for _, m := range r.modules {
+		if !m.enabled {
+			continue
+		}
+		for _, dep := range m.DependsOn {
+			if dm, ok := r.modules[dep]; ok && !dm.enabled {
+				return fmt.Errorf("module %q is enabled but its dependency %q is disabled", m.ID, dep)
+			}
+		}
+	}
+	return nil
+}
diff --git a/internal/modules/modules_test.go b/internal/modules/modules_test.go
new file mode 100644
index 0000000..1747591
--- /dev/null
+++ b/internal/modules/modules_test.go
@@ -0,0 +1,169 @@
+package modules
+
+import (
+	"testing"
+)
+
+func TestNewRegistry_DefaultState(t *testing.T) {
+	r := NewRegistry()
+
+	// Required modules are always enabled.
+	for _, id := range []string{Core, Schemas, Storage} {
+		if !r.IsEnabled(id) {
+			t.Errorf("required module %q should be enabled by default", id)
+		}
+	}
+
+	// Optional modules with DefaultEnabled=true.
+	for _, id := range []string{Auth, Projects, Audit, FreeCAD} {
+		if !r.IsEnabled(id) {
+			t.Errorf("module %q should be enabled by default", id)
+		}
+	}
+
+	// Optional modules with DefaultEnabled=false.
+	for _, id := range []string{Odoo, Jobs, DAG} {
+		if r.IsEnabled(id) {
+			t.Errorf("module %q should be disabled by default", id)
+		}
+	}
+}
+
+func TestSetEnabled_BasicToggle(t *testing.T) {
+	r := NewRegistry()
+
+	// Disable an optional module with no dependents.
+	if err := r.SetEnabled(Projects, false); err != nil {
+		t.Fatalf("disabling projects: %v", err)
+	}
+	if r.IsEnabled(Projects) {
+		t.Error("projects should be disabled after SetEnabled(false)")
+	}
+
+	// Re-enable it.
+	if err := r.SetEnabled(Projects, true); err != nil {
+		t.Fatalf("enabling projects: %v", err)
+	}
+	if !r.IsEnabled(Projects) {
+		t.Error("projects should be enabled after SetEnabled(true)")
+	}
+}
+
+func TestCannotDisableRequired(t *testing.T) {
+	r := NewRegistry()
+
+	for _, id := range []string{Core, Schemas, Storage} {
+		if err := r.SetEnabled(id, false); err == nil {
+			t.Errorf("disabling required module %q should return error", id)
+		}
+	}
+}
+
+func TestDependencyChain_EnableWithoutDep(t *testing.T) {
+	r := NewRegistry()
+
+	// Jobs depends on Auth. Auth is enabled by default, so enabling jobs works.
+	if err := r.SetEnabled(Jobs, true); err != nil {
+		t.Fatalf("enabling jobs (auth enabled): %v", err)
+	}
+
+	// DAG depends on Jobs. Jobs is now enabled, so enabling dag works.
+	if err := r.SetEnabled(DAG, true); err != nil {
+		t.Fatalf("enabling dag (jobs enabled): %v", err)
+	}
+
+	// Now try with deps disabled. Start fresh.
+	r2 := NewRegistry()
+
+	// DAG depends on Jobs, which is disabled by default.
+	if err := r2.SetEnabled(DAG, true); err == nil {
+		t.Error("enabling dag without jobs should fail")
+	}
+}
+
+func TestDisableDependedOn(t *testing.T) {
+	r := NewRegistry()
+
+	// Enable the full chain: auth (already on) → jobs → dag.
+	if err := r.SetEnabled(Jobs, true); err != nil {
+		t.Fatal(err)
+	}
+	if err := r.SetEnabled(DAG, true); err != nil {
+		t.Fatal(err)
+	}
+
+	// Cannot disable jobs while dag depends on it.
+	if err := r.SetEnabled(Jobs, false); err == nil {
+		t.Error("disabling jobs while dag is enabled should fail")
+	}
+
+	// Disable dag first, then jobs should work.
+	if err := r.SetEnabled(DAG, false); err != nil {
+		t.Fatal(err)
+	}
+	if err := r.SetEnabled(Jobs, false); err != nil {
+		t.Fatalf("disabling jobs after dag disabled: %v", err)
+	}
+}
+
+func TestCannotDisableAuthWhileJobsEnabled(t *testing.T) {
+	r := NewRegistry()
+
+	if err := r.SetEnabled(Jobs, true); err != nil {
+		t.Fatal(err)
+	}
+
+	// Auth is depended on by jobs.
+	if err := r.SetEnabled(Auth, false); err == nil {
+		t.Error("disabling auth while jobs is enabled should fail")
+	}
+}
+
+func TestUnknownModule(t *testing.T) {
+	r := NewRegistry()
+
+	if r.IsEnabled("nonexistent") {
+		t.Error("unknown module should not be enabled")
+	}
+	if err := r.SetEnabled("nonexistent", true); err == nil {
+		t.Error("setting unknown module should return error")
+	}
+	if r.Get("nonexistent") != nil {
+		t.Error("getting unknown module should return nil")
+	}
+}
+
+func TestAll_ReturnsAllModules(t *testing.T) {
+	r := NewRegistry()
+	all := r.All()
+
+	if len(all) != 10 {
+		t.Errorf("expected 10 modules, got %d", len(all))
+	}
+
+	// Should be sorted by ID.
+	for i := 1; i < len(all); i++ {
+		if all[i].ID < all[i-1].ID {
+			t.Errorf("modules not sorted: %s before %s", all[i-1].ID, all[i].ID)
+		}
+	}
+}
+
+func TestValidateDependencies(t *testing.T) {
+	r := NewRegistry()
+
+	// Default state should be valid.
+	if err := r.ValidateDependencies(); err != nil {
+		t.Fatalf("default state should be valid: %v", err)
+	}
+
+	// Force an invalid state by directly mutating (bypassing SetEnabled).
+	r.mu.Lock()
+	r.modules[Jobs].enabled = true
+	r.modules[Auth].enabled = false
+	r.mu.Unlock()
+
+	if err := r.ValidateDependencies(); err == nil {
+		t.Error("should detect jobs enabled without auth")
+	}
+}