diff --git a/src/relationship.ts b/src/relationship.ts
index 27d454a..309309e 100644
--- a/src/relationship.ts
+++ b/src/relationship.ts
@@ -1,24 +1,55 @@
 // ── Relationship ─────────────────────────────────────
 /**
  * A relationship definition — like a component, but represents a directed
- * link between two entities.
+ * link between two entities. Every relationship carries an optional data
+ * payload (defaults to `{}` for bare edges).
  *
  * @example
  * ```ts
  * const ChildOf = defineRelationship('childOf');
- * world.relate(child, ChildOf, parent);
+ * const Health  = defineRelationship('health', { hp: 100 });
  * ```
  */
-export interface RelationshipDef {
+export interface RelationshipDef<T extends Record<string, any> = {}> {
   /** Unique symbol used as the storage key. */
   readonly _key: symbol;
   /** Human-readable name, used for serialization. */
   readonly name: string;
+  /** Default values used when no data override is provided. */
+  readonly defaults: T;
+  /** Phantom type for inference. */
+  readonly type: T;
 }
 
 /**
  * Define a named relationship between entities.
+ *
+ * When `defaults` is omitted the relationship is a bare edge (no data).
+ * When `defaults` is provided the relationship carries data accessible
+ * via `world.getRelData()` / `world.setRelData()`.
+ *
+ * @example
+ * ```ts
+ * // Bare edge
+ * const ChildOf = defineRelationship('childOf');
+ *
+ * // With data
+ * const Health = defineRelationship('health', { hp: 100 });
+ * ```
  */
-export function defineRelationship(name: string): RelationshipDef {
-  return { _key: Symbol(), name };
+export function defineRelationship(name: string): RelationshipDef<{}>;
+export function defineRelationship<T extends Record<string, any>>(
+  name: string,
+  defaults: T,
+): RelationshipDef<T>;
+export function defineRelationship<T extends Record<string, any>>(
+  name: string,
+  defaults?: T,
+): RelationshipDef<{}> | RelationshipDef<T> {
+  return {
+    _key: Symbol(),
+    name,
+    defaults: (defaults ?? {}) as any,
+    type: undefined as unknown as any,
+  };
 }
diff --git a/src/serialization.ts b/src/serialization.ts
index 6b28567..07e86ec 100644
--- a/src/serialization.ts
+++ b/src/serialization.ts
@@ -5,6 +5,9 @@
 export interface WorldSnapshot {
   /** Entity stable ID → component map (component name → data). */
   entities: Record<string, Record<string, unknown>>;
-  /** Relationship name → (source ID → target ID). */
-  relationships: Record<string, Record<string, string>>;
+  /** Relationship name → (source ID → target ID or edge object). */
+  relationships: Record<
+    string,
+    Record<string, string | { target: string; data: unknown }>
+  >;
 }
diff --git a/src/world.ts b/src/world.ts
index e4756b1..82a18c6 100644
--- a/src/world.ts
+++ b/src/world.ts
@@ -35,6 +35,8 @@ export class World {
   private _relReverse = new Map<symbol, Map<number, Set<number>>>();
   /** Key → RelationshipDef for event emission. */
   private _relKeyToDef = new Map<symbol, RelationshipDef>();
+  /** Relationship data: relationship._key → SparseSet<data> (keyed by source index). */
+  private _relData = new Map<symbol, SparseSet<any>>();
 
   // ── Change tracking ───────────────────────────────
   private _dirty = new Map<symbol, Set<number>>();
@@ -93,6 +95,8 @@ export class World {
       if (fwd.has(idx)) {
         const target = fwd.get(idx);
         const rel = this._relKeyToDef.get(key)!;
+        // Clean up relationship data if applicable
+        this._relData.get(key)?.remove(idx);
         this._relRemoveEdge(entity, target, rel);
       }
 
@@ -322,8 +326,17 @@ export class World {
    * Create a directed relationship from `source` to `target`.
    * Each source can only have one target per relationship type.
    * If a relationship already exists, it is replaced.
+   *
+   * An optional `data` payload can be provided to store data along
+   * with the edge (accessible via `getRelData` / `setRelData`).
+   * Data is stored lazily — bare edges without data use no storage.
    */
-  relate(source: Entity, rel: RelationshipDef, target: Entity): void {
+  relate<T extends Record<string, any> = {}>(
+    source: Entity,
+    rel: RelationshipDef<T>,
+    target: Entity,
+    data?: Partial<T>,
+  ): void {
     const si = entityIndex(source);
     const ti = entityIndex(target);
     this._assertAlive(si, source);
@@ -351,6 +364,16 @@ export class World {
     this._relCounts[si]++;
     this._relCounts[ti]++;
 
+    // Lazy data storage — only allocate when data is provided
+    if (data !== undefined) {
+      let dataStore = this._relData.get(rel._key);
+      if (!dataStore) {
+        dataStore = new SparseSet<any>();
+        this._relData.set(rel._key, dataStore);
+      }
+      dataStore.set(si, { ...rel.defaults, ...data });
+    }
+
     this._emit({
       type: "relationshipAdded",
       source,
@@ -370,9 +393,48 @@ export class World {
     const target = this.getRelated(source, rel);
     if (target === undefined) return;
 
+    this._relData.get(rel._key)?.remove(si);
     this._relRemoveEdge(source, target, rel);
   }
 
+  /**
+   * Get the data stored alongside a relationship.
+   * Returns the relationship's defaults if no data was explicitly set.
+   */
+  getRelData<T extends Record<string, any> = {}>(
+    source: Entity,
+    rel: RelationshipDef<T>,
+  ): T {
+    const si = entityIndex(source);
+    this._assertAlive(si, source);
+
+    const store = this._relData.get(rel._key);
+    if (!store || !store.has(si)) {
+      return { ...rel.defaults };
+    }
+    return store.get(si);
+  }
+
+  /**
+   * Set the data for a relationship edge.
+   * Creates storage lazily if this is the first data set on this relationship type.
+   */
+  setRelData<T extends Record<string, any> = {}>(
+    source: Entity,
+    rel: RelationshipDef<T>,
+    data: T,
+  ): void {
+    const si = entityIndex(source);
+    this._assertAlive(si, source);
+
+    let store = this._relData.get(rel._key);
+    if (!store) {
+      store = new SparseSet<any>();
+      this._relData.set(rel._key, store);
+    }
+    store.set(si, data);
+  }
+
   /**
    * Get the target entity for a relationship, or undefined.
    */
@@ -505,14 +567,26 @@ export class World {
     }
 
     // Relationships
-    const relationships: Record<string, Record<string, string>> = {};
+    const relationships: Record<
+      string,
+      Record<string, string | { target: string; data: unknown }>
+    > = {};
     for (const [key, fwd] of this._relForward) {
       const rel = this._relKeyToDef.get(key)!;
-      const edges: Record<string, string> = {};
+      const edges: Record<string, string | { target: string; data: unknown }> =
+        {};
+      const dataStore = this._relData.get(key);
       for (const [si, target] of fwd.entries()) {
         const ti = entityIndex(target);
         if (ids[si] !== undefined && ids[ti] !== undefined) {
-          edges[ids[si]] = ids[ti];
+          if (dataStore?.has(si)) {
+            edges[ids[si]] = {
+              target: ids[ti],
+              data: dataStore.get(si),
+            };
+          } else {
+            edges[ids[si]] = ids[ti];
+          }
         }
       }
       if (Object.keys(edges).length > 0) {
@@ -572,11 +646,23 @@ export class World {
             `Pass it in the relationships array.`,
         );
       }
-      for (const [srcId, tgtId] of Object.entries(edges)) {
+      for (const [srcId, value] of Object.entries(edges)) {
         const source = idToEntity.get(srcId);
-        const target = idToEntity.get(tgtId);
-        if (source && target) {
-          world.relate(source, rel, target);
+        if (!source) continue;
+
+        if (typeof value === "string") {
+          // Pure edge (no data)
+          const target = idToEntity.get(value);
+          if (target) {
+            world.relate(source, rel, target);
+          }
+        } else if (typeof value === "object" && value !== null) {
+          // Edge with data
+          const edge = value as { target: string; data?: unknown };
+          const target = idToEntity.get(edge.target);
+          if (target) {
+            world.relate(source, rel, target, edge.data as any);
+          }
         }
       }
     }
diff --git a/test/relationships.test.ts b/test/relationships.test.ts
index 33340a4..2954727 100644
--- a/test/relationships.test.ts
+++ b/test/relationships.test.ts
@@ -379,3 +379,156 @@ describe("Dead entity safety", () => {
     expect([...world.getRelatedTo(e, ChildOf)]).toEqual([]);
   });
 });
+
+// ── Data-carrying relationships ───────────────────────
+describe("Data-carrying relationships", () => {
+  let world: World;
+
+  beforeEach(() => {
+    world = new World();
+  });
+
+  it("defines a data-carrying relationship", () => {
+    const Health = defineRelationship("health", { hp: 100, maxHp: 100 });
+    expect(Health.name).toBe("health");
+    expect(Health.defaults).toEqual({ hp: 100, maxHp: 100 });
+  });
+
+  it("relate stores defaults as data", () => {
+    const Health = defineRelationship("health", { hp: 100 });
+    const player = world.spawn();
+    const game = world.spawn();
+
+    world.relate(player, Health, game);
+    const data = world.getRelData(player, Health);
+    expect(data).toEqual({ hp: 100 });
+  });
+
+  it("relate accepts data override", () => {
+    const Health = defineRelationship("health", { hp: 100 });
+    const player = world.spawn();
+    const game = world.spawn();
+
+    world.relate(player, Health, game, { hp: 50 });
+    const data = world.getRelData(player, Health);
+    expect(data).toEqual({ hp: 50 });
+  });
+
+  it("setRelData updates relationship data", () => {
+    const Health = defineRelationship("health", { hp: 100 });
+    const player = world.spawn();
+    const game = world.spawn();
+
+    world.relate(player, Health, game);
+    world.setRelData(player, Health, { hp: 75 });
+    expect(world.getRelData(player, Health)).toEqual({ hp: 75 });
+  });
+
+  it("getRelData returns defaults when no data was set", () => {
+    const Health = defineRelationship("health", { hp: 100 });
+    const player = world.spawn();
+
+    // Even without an edge, getRelData returns a copy of defaults
+    expect(world.getRelData(player, Health)).toEqual({ hp: 100 });
+  });
+
+  it("setRelData works even without prior relate", () => {
+    const Health = defineRelationship("health", { hp: 100 });
+    const player = world.spawn();
+
+    world.setRelData(player, Health, { hp: 50 });
+    expect(world.getRelData(player, Health)).toEqual({ hp: 50 });
+  });
+
+  it("data survives unrelate and re-relate", () => {
+    const Health = defineRelationship("health", { hp: 100 });
+    const player = world.spawn();
+    const game = world.spawn();
+
+    world.relate(player, Health, game, { hp: 50 });
+    world.unrelate(player, Health);
+
+    // After unrelate, stored data is gone, returns defaults
+    expect(world.getRelData(player, Health)).toEqual({ hp: 100 });
+
+    world.relate(player, Health, game, { hp: 80 });
+    // Note: this is a *new* relate with data, so stored data is { hp: 80 }
+    expect(world.getRelData(player, Health)).toEqual({ hp: 80 });
+  });
+
+  it("data is cleaned up on destroy", () => {
+    const Health = defineRelationship("health", { hp: 100 });
+    const player = world.spawn();
+    const game = world.spawn();
+
+    world.relate(player, Health, game, { hp: 50 });
+    world.destroy(player);
+
+    // After destroy the entity is dead — assertAlive throws, not the data lookup
+    expect(() => world.getRelData(player, Health)).toThrow("not alive");
+  });
+
+  it("setRelData with no prior edge stores data that getsRelated does not see", () => {
+    const Health = defineRelationship("health", { hp: 100 });
+    const player = world.spawn();
+
+    world.setRelData(player, Health, { hp: 50 });
+    // No edge exists yet
+    expect(world.getRelated(player, Health)).toBeUndefined();
+    // But data is stored (decoupled storage)
+    expect(world.getRelData(player, Health)).toEqual({ hp: 50 });
+  });
+
+  it("data-carrying relationships serialize and deserialize", () => {
+    const Health = defineRelationship("health", { hp: 100, maxHp: 100 });
+    const player = world.spawn();
+    const game = world.spawn();
+
+    world.relate(player, Health, game, { hp: 50, maxHp: 100 });
+
+    const snapshot = world.toJSON();
+
+    // Verify the snapshot has data in the relationship structure
+    const relSection = snapshot.relationships["health"];
+    expect(relSection).toBeDefined();
+
+    // Find the player source ID — it's the entity without components
+    const playerId = Object.keys(snapshot.entities).find(
+      (id) => Object.keys(snapshot.entities[id]).length === 0,
+    )!;
+    const edgeValue = relSection[playerId];
+    expect(typeof edgeValue).toBe("object");
+    expect((edgeValue as any).target).toBeDefined();
+    expect((edgeValue as any).data).toEqual({ hp: 50, maxHp: 100 });
+
+    // Check JSON round-trip preserves data
+    const parsed = JSON.parse(JSON.stringify(snapshot));
+    const world2 = World.fromJSON(parsed, [], [Health]);
+
+    const snap2 = world2.toJSON();
+    const playerId2 = Object.keys(snap2.entities).find(
+      (id) => Object.keys(snap2.entities[id]).length === 0,
+    )!;
+    expect(snap2.relationships["health"]).toBeDefined();
+    const edgeValue2 = snap2.relationships["health"][playerId2];
+    expect((edgeValue2 as any).data).toEqual({ hp: 50, maxHp: 100 });
+  });
+
+  it("pure edge-defined relationships still work alongside data relationships", () => {
+    const ChildOf2 = defineRelationship("childOf2");
+    const Score = defineRelationship("score", { points: 0 });
+
+    const parent = world.spawn();
+    const child = world.spawn();
+    const game = world.spawn();
+
+    world.relate(child, ChildOf2, parent);
+    world.relate(child, Score, game, { points: 42 });
+
+    // Pure edge still works
+    expect(world.getRelated(child, ChildOf2)).toBe(parent);
+
+    // Data edge still works
+    expect(world.getRelData(child, Score)).toEqual({ points: 42 });
+  });
+});