diff --git a/src/db/repositories/drizzle/phase.ts b/src/db/repositories/drizzle/phase.ts
index b5cd53a..abfb869 100644
--- a/src/db/repositories/drizzle/phase.ts
+++ b/src/db/repositories/drizzle/phase.ts
@@ -7,7 +7,7 @@
 import { eq, asc, and, max } from 'drizzle-orm';
 import { nanoid } from 'nanoid';
 import type { DrizzleDatabase } from '../../index.js';
-import { phases, type Phase } from '../../schema.js';
+import { phases, phaseDependencies, type Phase } from '../../schema.js';
 import type {
   PhaseRepository,
   CreatePhaseData,
@@ -102,4 +102,34 @@ export class DrizzlePhaseRepository implements PhaseRepository {
 
     await this.db.delete(phases).where(eq(phases.id, id));
   }
+
+  async createDependency(phaseId: string, dependsOnPhaseId: string): Promise<void> {
+    const id = nanoid();
+    const now = new Date();
+
+    await this.db.insert(phaseDependencies).values({
+      id,
+      phaseId,
+      dependsOnPhaseId,
+      createdAt: now,
+    });
+  }
+
+  async getDependencies(phaseId: string): Promise<string[]> {
+    const result = await this.db
+      .select({ dependsOnPhaseId: phaseDependencies.dependsOnPhaseId })
+      .from(phaseDependencies)
+      .where(eq(phaseDependencies.phaseId, phaseId));
+
+    return result.map((row) => row.dependsOnPhaseId);
+  }
+
+  async getDependents(phaseId: string): Promise<string[]> {
+    const result = await this.db
+      .select({ phaseId: phaseDependencies.phaseId })
+      .from(phaseDependencies)
+      .where(eq(phaseDependencies.dependsOnPhaseId, phaseId));
+
+    return result.map((row) => row.phaseId);
+  }
 }
diff --git a/src/db/repositories/phase-repository.ts b/src/db/repositories/phase-repository.ts
index 9ebb601..b9d7e82 100644
--- a/src/db/repositories/phase-repository.ts
+++ b/src/db/repositories/phase-repository.ts
@@ -71,4 +71,23 @@ export interface PhaseRepository {
    * Cascades to child plans and tasks via FK constraints.
    */
   delete(id: string): Promise<void>;
+
+  /**
+   * Create a dependency between two phases.
+   * The phase identified by phaseId will depend on dependsOnPhaseId.
+   * Both phases must exist.
+   */
+  createDependency(phaseId: string, dependsOnPhaseId: string): Promise<void>;
+
+  /**
+   * Get IDs of phases that this phase depends on.
+   * Returns empty array if no dependencies.
+   */
+  getDependencies(phaseId: string): Promise<string[]>;
+
+  /**
+   * Get IDs of phases that depend on this phase.
+   * Returns empty array if no dependents.
+   */
+  getDependents(phaseId: string): Promise<string[]>;
 }