diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
new file mode 100644
index 0000000..8b9157f
--- /dev/null
+++ b/.github/workflows/tests.yml
@@ -0,0 +1,63 @@
+name: CI Tests
+
+on:
+  push:
+    branches: [ main, development, testing ]
+  pull_request:
+    branches: [ main, development, testing ]
+
+jobs:
+  backend-tests:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout repo
+        uses: actions/checkout@v4
+
+      - name: Setup Node
+        uses: actions/setup-node@v4
+        with:
+          node-version: 20
+
+      - name: Install backend deps
+        working-directory: backend
+        run: npm ci
+
+      - name: Run backend unit tests
+        working-directory: backend
+        run: npm run test
+
+      - name: Run backend e2e tests
+        working-directory: backend
+        env:
+          SUPABASE_URL: ${{ secrets.SUPABASE_URL }}
+          SUPABASE_SERVICE_ROLE_KEY: ${{ secrets.SUPABASE_SERVICE_ROLE_KEY }}
+          SUPABASE_ANON_KEY: ${{ secrets.SUPABASE_ANON_KEY }}
+        run: |
+          cat <<EOF > .env.local
+SUPABASE_URL=${SUPABASE_URL}
+SUPABASE_SERVICE_ROLE_KEY=${SUPABASE_SERVICE_ROLE_KEY}
+SUPABASE_ANON_KEY=${SUPABASE_ANON_KEY}
+PORT=3000
+EOF
+          npm run test:e2e
+
+  frontend-tests:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout repo
+        uses: actions/checkout@v4
+
+      - name: Setup Node
+        uses: actions/setup-node@v4
+        with:
+          node-version: 20
+
+      - name: Install frontend deps
+        working-directory: frontend
+        run: npm ci
+
+      - name: Run frontend tests
+        working-directory: frontend
+        run: npm run test
\ No newline at end of file
diff --git a/backend/src/auth/auth.service.spec.ts b/backend/src/auth/auth.service.spec.ts
new file mode 100644
index 0000000..4cd5c84
--- /dev/null
+++ b/backend/src/auth/auth.service.spec.ts
@@ -0,0 +1,104 @@
+import {
+  BadRequestException,
+  ConflictException,
+  InternalServerErrorException,
+  UnauthorizedException,
+} from '@nestjs/common';
+import { AuthService } from './auth.service';
+
+// Minimal SupabaseService shape for mocking
+interface MockSupabaseClient {
+  from: jest.Mock;
+  auth: {
+    signUp: jest.Mock;
+    signInWithPassword: jest.Mock;
+  };
+}
+
+describe('AuthService (unit)', () => {
+  let service: AuthService;
+  let mockSupabaseClient: MockSupabaseClient;
+  let mockSupabaseService: {
+    getClient: jest.Mock;
+    getAnonClient: jest.Mock;
+  };
+
+  beforeEach(() => {
+    mockSupabaseClient = {
+      from: jest.fn(),
+      auth: {
+        signUp: jest.fn(),
+        signInWithPassword: jest.fn(),
+      },
+    };
+
+    mockSupabaseService = {
+      getClient: jest.fn(() => mockSupabaseClient),
+      getAnonClient: jest.fn(() => mockSupabaseClient),
+    };
+
+    service = new AuthService(mockSupabaseService as any);
+  });
+
+  describe('signup', () => {
+    const baseSignupDto = {
+      email: 'TestUser@example.com',
+      password: 'Test1234!',
+      displayName: 'TestUser',
+      firstName: 'Test',
+      lastName: 'User',
+    };
+
+    it('throws ConflictException when display name is already taken', async () => {
+      // Mock supabase.from('users').select(...) to return an existing user
+      mockSupabaseClient.from.mockImplementation((table: string) => {
+        if (table === 'users') {
+          return {
+            select: jest.fn().mockResolvedValue({
+              data: [{ display_name: 'testuser' }],
+              error: null,
+            }),
+          } as any;
+        }
+        return { select: jest.fn().mockResolvedValue({ data: [], error: null }) } as any;
+      });
+
+      await expect(service.signup(baseSignupDto as any)).rejects.toThrow(
+        ConflictException,
+      );
+      await expect(service.signup(baseSignupDto as any)).rejects.toThrow(
+        'This display name is already taken. Please choose a different name.',
+      );
+    });
+
+    it('throws ConflictException when email is already registered', async () => {
+      // 1) No duplicate display name
+      mockSupabaseClient.from.mockImplementation((table: string) => {
+        if (table === 'users') {
+          return {
+            select: jest.fn().mockResolvedValue({
+              data: [{ display_name: 'someone-else' }],
+              error: null,
+            }),
+          } as any;
+        }
+        return { select: jest.fn().mockResolvedValue({ data: [], error: null }) } as any;
+      });
+
+      // 2) Supabase auth.signUp returns an "already registered" error
+      mockSupabaseClient.auth.signUp.mockResolvedValueOnce({
+        data: { user: null, session: null },
+        error: { message: 'already registered' },
+      });
+
+      const promise = service.signup(baseSignupDto as any);
+
+      await expect(promise).rejects.toThrow(ConflictException);
+      await expect(promise).rejects.toThrow(
+        'An account with this email already exists.',
+      );
+    });
+  });
+});
+
+
diff --git a/backend/test/app.e2e-spec.ts b/backend/test/app.e2e-spec.ts
index b16c798..07e3d2e 100644
--- a/backend/test/app.e2e-spec.ts
+++ b/backend/test/app.e2e-spec.ts
@@ -1,16 +1,18 @@
-import { Test, TestingModule } from '@nestjs/testing';
 import { INestApplication } from '@nestjs/common';
+import { Test, TestingModule } from '@nestjs/testing';
+import { config } from 'dotenv';
+import { join } from 'path';
 import request from 'supertest';
 import { App } from 'supertest/types';
 import { AppModule } from './../src/app.module';
-import { config } from 'dotenv';
-import { join } from 'path';
 
-describe('AppController (e2e)', () => {
+describe('App e2e', () => {
   let app: INestApplication<App>;
 
   beforeEach(async () => {
+    // Load test environment (points at test Supabase project, etc.)
     config({ path: join(__dirname, '..', '.env.local') });
+
     const moduleFixture: TestingModule = await Test.createTestingModule({
       imports: [AppModule],
     }).compile();
@@ -29,4 +31,48 @@ describe('AppController (e2e)', () => {
       .expect(200)
       .expect('Hello World!');
   });
+
+  it('Flow 1: signup -> login -> GET /circles returns empty list for new user', async () => {
+    const server = app.getHttpServer();
+
+    const ts = Date.now();
+    const email = `qa-user-${ts}@example.com`;
+    const password = 'Test1234!';
+    const displayName = `qa_user_${ts}`;
+    const firstName = 'QA';
+    const lastName = 'User';
+
+    // 1) Sign up a brand new user
+    await request(server)
+      .post('/auth/signup')
+      .send({
+        email,
+        password,
+        displayName,
+        firstName,
+        lastName,
+      })
+      .expect(201);
+
+    // 2) Log in with the same credentials to get a Supabase access token
+    const loginRes = await request(server)
+      .post('/auth/login')
+      .send({
+        identifier: email,
+        password,
+      })
+      .expect(200);
+
+    const accessToken = loginRes.body?.session?.access_token;
+    expect(accessToken).toBeDefined();
+
+    // 3) Call /circles with the Bearer token and expect an empty array
+    const circlesRes = await request(server)
+      .get('/circles')
+      .set('Authorization', `Bearer ${accessToken}`)
+      .expect(200);
+
+    expect(Array.isArray(circlesRes.body)).toBe(true);
+    expect(circlesRes.body.length).toBe(0);
+  });
 });