diff --git a/internal/module/module.go b/internal/module/module.go
new file mode 100644
index 0000000..a286066
--- /dev/null
+++ b/internal/module/module.go
@@ -0,0 +1,61 @@
+// Package module defines the four-phase contract every installable unit
+// implements, along with the value types exchanged with the engine.
+package module
+
+import "context"
+
+// Meta is static, declarative information about a module.
+type Meta struct {
+	ID          string   // stable machine identifier, e.g. "packages"
+	Name        string   // human label, e.g. "Base packages"
+	Description string   // one-line summary for TUI
+	Deps        []string // IDs of modules that must complete before this one
+}
+
+// State is an opaque, module-defined snapshot of current system state.
+// The engine never inspects it; it is threaded from Detect → Plan.
+type State any
+
+// ActionKind categorizes what an Action does, for UI grouping only.
+type ActionKind int
+
+const (
+	ActionUnknown ActionKind = iota
+	ActionPackageInstall
+	ActionFileWrite
+	ActionSymlink
+	ActionCommand
+	ActionGitClone
+	ActionGroupAdd
+)
+
+// Action is a single planned change with a human-readable description.
+type Action struct {
+	Description string
+	Kind        ActionKind
+	Exec        func(ctx context.Context) error
+}
+
+// Plan is an ordered list of actions returned by Module.Plan.
+type Plan struct {
+	Actions []Action
+}
+
+// IsEmpty reports whether the plan would make no changes.
+func (p Plan) IsEmpty() bool { return len(p.Actions) == 0 }
+
+// Module is the four-phase contract.
+type Module interface {
+	Meta() Meta
+	Detect(ctx context.Context) (State, error)
+	Plan(ctx context.Context, current State) (Plan, error)
+	Apply(ctx context.Context, plan Plan, r Reporter) error
+	Verify(ctx context.Context) error
+}
+
+// Reporter is how modules stream progress events. Defined here as an
+// interface to avoid an import cycle with the engine package.
+type Reporter interface {
+	Progress(moduleID string, current, total int, message string)
+	Log(moduleID, line string)
+}
diff --git a/internal/module/registry.go b/internal/module/registry.go
new file mode 100644
index 0000000..303f725
--- /dev/null
+++ b/internal/module/registry.go
@@ -0,0 +1,56 @@
+package module
+
+import (
+	"fmt"
+	"sort"
+	"sync"
+)
+
+// Registry holds all known modules keyed by Meta.ID.
+type Registry struct {
+	mu      sync.RWMutex
+	modules map[string]Module
+}
+
+// NewRegistry returns an empty registry.
+func NewRegistry() *Registry {
+	return &Registry{modules: make(map[string]Module)}
+}
+
+// Register adds a module. Returns an error if the ID is already present
+// or empty. Safe for concurrent use.
+func (r *Registry) Register(m Module) error {
+	id := m.Meta().ID
+	if id == "" {
+		return fmt.Errorf("module has empty ID")
+	}
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	if _, dup := r.modules[id]; dup {
+		return fmt.Errorf("module %q already registered", id)
+	}
+	r.modules[id] = m
+	return nil
+}
+
+// Get returns the module with the given ID, or (nil, false) if absent.
+func (r *Registry) Get(id string) (Module, bool) {
+	r.mu.RLock()
+	defer r.mu.RUnlock()
+	m, ok := r.modules[id]
+	return m, ok
+}
+
+// All returns all registered modules sorted by ID for determinism.
+func (r *Registry) All() []Module {
+	r.mu.RLock()
+	defer r.mu.RUnlock()
+	out := make([]Module, 0, len(r.modules))
+	for _, m := range r.modules {
+		out = append(out, m)
+	}
+	sort.Slice(out, func(i, j int) bool {
+		return out[i].Meta().ID < out[j].Meta().ID
+	})
+	return out
+}
diff --git a/internal/module/registry_test.go b/internal/module/registry_test.go
new file mode 100644
index 0000000..3536051
--- /dev/null
+++ b/internal/module/registry_test.go
@@ -0,0 +1,57 @@
+package module
+
+import (
+	"context"
+	"testing"
+)
+
+type fakeModule struct{ id string; deps []string }
+
+func (f fakeModule) Meta() Meta { return Meta{ID: f.id, Name: f.id, Deps: f.deps} }
+func (f fakeModule) Detect(context.Context) (State, error)              { return nil, nil }
+func (f fakeModule) Plan(context.Context, State) (Plan, error)          { return Plan{}, nil }
+func (f fakeModule) Apply(context.Context, Plan, Reporter) error        { return nil }
+func (f fakeModule) Verify(context.Context) error                       { return nil }
+
+func TestRegistry(t *testing.T) {
+	t.Run("register then get", func(t *testing.T) {
+		r := NewRegistry()
+		if err := r.Register(fakeModule{id: "a"}); err != nil {
+			t.Fatal(err)
+		}
+		m, ok := r.Get("a")
+		if !ok || m.Meta().ID != "a" {
+			t.Fatalf("want module a, got %v ok=%v", m, ok)
+		}
+	})
+
+	t.Run("duplicate register errors", func(t *testing.T) {
+		r := NewRegistry()
+		_ = r.Register(fakeModule{id: "a"})
+		if err := r.Register(fakeModule{id: "a"}); err == nil {
+			t.Fatal("expected duplicate error")
+		}
+	})
+
+	t.Run("empty id errors", func(t *testing.T) {
+		r := NewRegistry()
+		if err := r.Register(fakeModule{id: ""}); err == nil {
+			t.Fatal("expected empty-id error")
+		}
+	})
+
+	t.Run("all is sorted by id", func(t *testing.T) {
+		r := NewRegistry()
+		_ = r.Register(fakeModule{id: "b"})
+		_ = r.Register(fakeModule{id: "a"})
+		_ = r.Register(fakeModule{id: "c"})
+		all := r.All()
+		got := []string{all[0].Meta().ID, all[1].Meta().ID, all[2].Meta().ID}
+		want := []string{"a", "b", "c"}
+		for i := range got {
+			if got[i] != want[i] {
+				t.Fatalf("All() = %v, want %v", got, want)
+			}
+		}
+	})
+}