diff --git a/backend/apps/api/src/api.module.ts b/backend/apps/api/src/api.module.ts
index dddd396..4699d94 100644
--- a/backend/apps/api/src/api.module.ts
+++ b/backend/apps/api/src/api.module.ts
@@ -5,9 +5,16 @@ import { AppService } from './app.service';
 import { HealthModule } from './health/health.module';
 import { VersionModule } from './version/version.module';
 import { UsersApiModule } from './users/users.module';
+import { TripsApiModule } from './trips/trips.module';
 
 @Module({
-  imports: [ConfigurationModule, HealthModule, VersionModule, UsersApiModule],
+  imports: [
+    ConfigurationModule,
+    HealthModule,
+    VersionModule,
+    UsersApiModule,
+    TripsApiModule,
+  ],
   controllers: [AppController],
   providers: [AppService],
 })
diff --git a/backend/apps/api/src/trips/trips.controller.spec.ts b/backend/apps/api/src/trips/trips.controller.spec.ts
new file mode 100644
index 0000000..47eb7e9
--- /dev/null
+++ b/backend/apps/api/src/trips/trips.controller.spec.ts
@@ -0,0 +1,73 @@
+import { TripsController } from './trips.controller';
+import type { AuthenticatedUser } from '../../../../libs/auth/src';
+
+describe('TripsController', () => {
+  const currentUser: AuthenticatedUser = {
+    id: 'u1',
+    externalSubjectId: 'sub-1',
+    displayName: 'Alex',
+    email: 'a@example.com',
+  };
+
+  it('GET /trips lists trips for the current user', async () => {
+    const tripsService = {
+      listTripsForUser: jest
+        .fn()
+        .mockResolvedValue([{ id: 't1', name: 'Slovenia 2027' }]),
+    };
+    const controller = new TripsController(tripsService as never);
+
+    await expect(controller.list(currentUser)).resolves.toEqual([
+      { id: 't1', name: 'Slovenia 2027' },
+    ]);
+    expect(tripsService.listTripsForUser).toHaveBeenCalledWith('u1');
+  });
+
+  it('POST /trips creates a trip owned by the current user', async () => {
+    const created = {
+      id: 't1',
+      name: 'Slovenia 2027',
+      ownerId: 'u1',
+      version: 1,
+    };
+    const tripsService = { createTrip: jest.fn().mockResolvedValue(created) };
+    const controller = new TripsController(tripsService as never);
+
+    await expect(
+      controller.create(currentUser, { name: 'Slovenia 2027' }),
+    ).resolves.toEqual(created);
+    expect(tripsService.createTrip).toHaveBeenCalledWith('u1', {
+      name: 'Slovenia 2027',
+    });
+  });
+
+  it('GET /trips/:tripId returns a single trip', async () => {
+    const trip = { id: 't1', name: 'Slovenia 2027' };
+    const tripsService = { getTrip: jest.fn().mockResolvedValue(trip) };
+    const controller = new TripsController(tripsService as never);
+
+    await expect(controller.getOne('t1')).resolves.toEqual(trip);
+  });
+
+  it('PATCH /trips/:tripId forwards the update dto including version', async () => {
+    const updated = { id: 't1', name: 'New name', version: 2 };
+    const tripsService = { updateTrip: jest.fn().mockResolvedValue(updated) };
+    const controller = new TripsController(tripsService as never);
+
+    await expect(
+      controller.update('t1', { name: 'New name', version: 1 }),
+    ).resolves.toEqual(updated);
+    expect(tripsService.updateTrip).toHaveBeenCalledWith('t1', {
+      name: 'New name',
+      version: 1,
+    });
+  });
+
+  it('DELETE /trips/:tripId deletes the trip', async () => {
+    const tripsService = { deleteTrip: jest.fn().mockResolvedValue(undefined) };
+    const controller = new TripsController(tripsService as never);
+
+    await controller.remove('t1');
+    expect(tripsService.deleteTrip).toHaveBeenCalledWith('t1');
+  });
+});
diff --git a/backend/apps/api/src/trips/trips.controller.ts b/backend/apps/api/src/trips/trips.controller.ts
new file mode 100644
index 0000000..254bf4d
--- /dev/null
+++ b/backend/apps/api/src/trips/trips.controller.ts
@@ -0,0 +1,56 @@
+import {
+  Body,
+  Controller,
+  Delete,
+  Get,
+  Param,
+  Patch,
+  Post,
+  UseGuards,
+} from '@nestjs/common';
+import { OidcAuthGuard } from '../../../../libs/auth/src';
+import type { AuthenticatedUser } from '../../../../libs/auth/src';
+import { TripsService } from '../../../../libs/trips/src';
+import type {
+  CreateTripDto,
+  Trip,
+  UpdateTripDto,
+} from '../../../../libs/trips/src';
+import { CurrentUser } from '../auth/current-user.decorator';
+
+@Controller('trips')
+@UseGuards(OidcAuthGuard)
+export class TripsController {
+  constructor(private readonly tripsService: TripsService) {}
+
+  @Get()
+  list(@CurrentUser() currentUser: AuthenticatedUser): Promise<Trip[]> {
+    return this.tripsService.listTripsForUser(currentUser.id);
+  }
+
+  @Post()
+  create(
+    @CurrentUser() currentUser: AuthenticatedUser,
+    @Body() dto: CreateTripDto,
+  ): Promise<Trip> {
+    return this.tripsService.createTrip(currentUser.id, dto);
+  }
+
+  @Get(':tripId')
+  getOne(@Param('tripId') tripId: string): Promise<Trip> {
+    return this.tripsService.getTrip(tripId);
+  }
+
+  @Patch(':tripId')
+  update(
+    @Param('tripId') tripId: string,
+    @Body() dto: UpdateTripDto,
+  ): Promise<Trip> {
+    return this.tripsService.updateTrip(tripId, dto);
+  }
+
+  @Delete(':tripId')
+  remove(@Param('tripId') tripId: string): Promise<void> {
+    return this.tripsService.deleteTrip(tripId);
+  }
+}
diff --git a/backend/apps/api/src/trips/trips.module.ts b/backend/apps/api/src/trips/trips.module.ts
new file mode 100644
index 0000000..cec2767
--- /dev/null
+++ b/backend/apps/api/src/trips/trips.module.ts
@@ -0,0 +1,10 @@
+import { Module } from '@nestjs/common';
+import { AuthModule } from '../../../../libs/auth/src';
+import { TripsLibModule } from '../../../../libs/trips/src';
+import { TripsController } from './trips.controller';
+
+@Module({
+  imports: [AuthModule, TripsLibModule],
+  controllers: [TripsController],
+})
+export class TripsApiModule {}
diff --git a/backend/libs/trips/src/index.ts b/backend/libs/trips/src/index.ts
new file mode 100644
index 0000000..29998d4
--- /dev/null
+++ b/backend/libs/trips/src/index.ts
@@ -0,0 +1,4 @@
+export * from './trip.types';
+export * from './trips.service';
+export * from './trip-settings.service';
+export * from './trips.module';
diff --git a/backend/libs/trips/src/trip-settings.repository.ts b/backend/libs/trips/src/trip-settings.repository.ts
new file mode 100644
index 0000000..bf3206a
--- /dev/null
+++ b/backend/libs/trips/src/trip-settings.repository.ts
@@ -0,0 +1,67 @@
+import { Inject, Injectable } from '@nestjs/common';
+import type { Kysely } from 'kysely';
+import { KYSELY_DB } from '../../database/src';
+import type { Database } from '../../database/src';
+import type {
+  ResearchDepth,
+  TripPlanningStyle,
+  TripSettings,
+  UpdateTripSettingsDto,
+} from './trip.types';
+
+function toTripSettings(row: {
+  trip_id: string;
+  web_research_enabled: boolean;
+  periodic_agent_review_enabled: boolean;
+  notification_email_enabled: boolean;
+  notification_push_enabled: boolean;
+  default_research_depth: string | null;
+  default_planning_style: string | null;
+}): TripSettings {
+  return {
+    tripId: row.trip_id,
+    webResearchEnabled: row.web_research_enabled,
+    periodicAgentReviewEnabled: row.periodic_agent_review_enabled,
+    notificationEmailEnabled: row.notification_email_enabled,
+    notificationPushEnabled: row.notification_push_enabled,
+    defaultResearchDepth: row.default_research_depth as ResearchDepth | null,
+    defaultPlanningStyle:
+      row.default_planning_style as TripPlanningStyle | null,
+  };
+}
+
+@Injectable()
+export class TripSettingsRepository {
+  constructor(@Inject(KYSELY_DB) private readonly db: Kysely<Database>) {}
+
+  async findByTripId(tripId: string): Promise<TripSettings | undefined> {
+    const row = await this.db
+      .selectFrom('trip_settings')
+      .selectAll()
+      .where('trip_id', '=', tripId)
+      .executeTakeFirst();
+    return row ? toTripSettings(row) : undefined;
+  }
+
+  async replace(
+    tripId: string,
+    dto: UpdateTripSettingsDto,
+  ): Promise<TripSettings> {
+    const row = await this.db
+      .updateTable('trip_settings')
+      .set({
+        web_research_enabled: dto.webResearchEnabled,
+        periodic_agent_review_enabled: dto.periodicAgentReviewEnabled,
+        notification_email_enabled: dto.notificationEmailEnabled,
+        notification_push_enabled: dto.notificationPushEnabled,
+        default_research_depth: dto.defaultResearchDepth,
+        default_planning_style: dto.defaultPlanningStyle,
+        updated_at: new Date().toISOString(),
+      })
+      .where('trip_id', '=', tripId)
+      .returningAll()
+      .executeTakeFirstOrThrow();
+
+    return toTripSettings(row);
+  }
+}
diff --git a/backend/libs/trips/src/trip-settings.service.ts b/backend/libs/trips/src/trip-settings.service.ts
new file mode 100644
index 0000000..36cf444
--- /dev/null
+++ b/backend/libs/trips/src/trip-settings.service.ts
@@ -0,0 +1,21 @@
+import { Injectable, NotFoundException } from '@nestjs/common';
+import { TripSettingsRepository } from './trip-settings.repository';
+import type { TripSettings, UpdateTripSettingsDto } from './trip.types';
+
+@Injectable()
+export class TripSettingsService {
+  constructor(private readonly repository: TripSettingsRepository) {}
+
+  async getSettings(tripId: string): Promise<TripSettings> {
+    const settings = await this.repository.findByTripId(tripId);
+    if (!settings) throw new NotFoundException('Trip settings not found');
+    return settings;
+  }
+
+  replaceSettings(
+    tripId: string,
+    dto: UpdateTripSettingsDto,
+  ): Promise<TripSettings> {
+    return this.repository.replace(tripId, dto);
+  }
+}
diff --git a/backend/libs/trips/src/trip.types.ts b/backend/libs/trips/src/trip.types.ts
new file mode 100644
index 0000000..254e3cb
--- /dev/null
+++ b/backend/libs/trips/src/trip.types.ts
@@ -0,0 +1,74 @@
+export type TripStatus =
+  | 'DRAFT'
+  | 'PLANNING'
+  | 'BOOKING'
+  | 'UPCOMING'
+  | 'ACTIVE'
+  | 'COMPLETED'
+  | 'ARCHIVED';
+
+export interface Trip {
+  id: string;
+  name: string;
+  description: string | null;
+  ownerId: string;
+  startDate: string | null;
+  endDate: string | null;
+  status: TripStatus;
+  planningStage: string | null;
+  currency: string;
+  version: number;
+  createdAt: Date;
+  updatedAt: Date;
+}
+
+export interface CreateTripDto {
+  name: string;
+  description?: string;
+  startDate?: string;
+  endDate?: string;
+  currency?: string;
+}
+
+export interface UpdateTripDto {
+  name?: string;
+  description?: string | null;
+  startDate?: string | null;
+  endDate?: string | null;
+  status?: TripStatus;
+  planningStage?: string | null;
+  currency?: string;
+  /** The caller's last-known version; required so stale concurrent writes are rejected. */
+  version: number;
+}
+
+export type TripPlanningStyle = 'RELAXED' | 'BALANCED' | 'PACKED';
+export type ResearchDepth = 'MINIMAL' | 'STANDARD' | 'THOROUGH';
+
+export interface TripSettings {
+  tripId: string;
+  webResearchEnabled: boolean;
+  periodicAgentReviewEnabled: boolean;
+  notificationEmailEnabled: boolean;
+  notificationPushEnabled: boolean;
+  defaultResearchDepth: ResearchDepth | null;
+  defaultPlanningStyle: TripPlanningStyle | null;
+}
+
+export interface UpdateTripSettingsDto {
+  webResearchEnabled: boolean;
+  periodicAgentReviewEnabled: boolean;
+  notificationEmailEnabled: boolean;
+  notificationPushEnabled: boolean;
+  defaultResearchDepth: ResearchDepth | null;
+  defaultPlanningStyle: TripPlanningStyle | null;
+}
+
+export const DEFAULT_TRIP_SETTINGS: Omit<TripSettings, 'tripId'> = {
+  webResearchEnabled: false,
+  periodicAgentReviewEnabled: false,
+  notificationEmailEnabled: true,
+  notificationPushEnabled: true,
+  defaultResearchDepth: null,
+  defaultPlanningStyle: null,
+};
diff --git a/backend/libs/trips/src/trips.module.ts b/backend/libs/trips/src/trips.module.ts
new file mode 100644
index 0000000..4099ad7
--- /dev/null
+++ b/backend/libs/trips/src/trips.module.ts
@@ -0,0 +1,18 @@
+import { Module } from '@nestjs/common';
+import { DatabaseModule } from '../../database/src';
+import { TripsRepository } from './trips.repository';
+import { TripsService } from './trips.service';
+import { TripSettingsRepository } from './trip-settings.repository';
+import { TripSettingsService } from './trip-settings.service';
+
+@Module({
+  imports: [DatabaseModule],
+  providers: [
+    TripsRepository,
+    TripsService,
+    TripSettingsRepository,
+    TripSettingsService,
+  ],
+  exports: [TripsService, TripSettingsService],
+})
+export class TripsLibModule {}
diff --git a/backend/libs/trips/src/trips.repository.ts b/backend/libs/trips/src/trips.repository.ts
new file mode 100644
index 0000000..6e3fe21
--- /dev/null
+++ b/backend/libs/trips/src/trips.repository.ts
@@ -0,0 +1,141 @@
+import { Inject, Injectable } from '@nestjs/common';
+import { sql } from 'kysely';
+import type { Kysely } from 'kysely';
+import { KYSELY_DB } from '../../database/src';
+import type { Database } from '../../database/src';
+import { DEFAULT_TRIP_SETTINGS } from './trip.types';
+import type { CreateTripDto, Trip, TripStatus } from './trip.types';
+
+function toTrip(row: {
+  id: string;
+  name: string;
+  description: string | null;
+  owner_id: string;
+  start_date: string | null;
+  end_date: string | null;
+  status: string;
+  planning_stage: string | null;
+  currency: string;
+  version: number;
+  created_at: Date;
+  updated_at: Date;
+}): Trip {
+  return {
+    id: row.id,
+    name: row.name,
+    description: row.description,
+    ownerId: row.owner_id,
+    startDate: row.start_date,
+    endDate: row.end_date,
+    status: row.status as TripStatus,
+    planningStage: row.planning_stage,
+    currency: row.currency,
+    version: row.version,
+    createdAt: row.created_at,
+    updatedAt: row.updated_at,
+  };
+}
+
+export interface TripUpdateFields {
+  name: string;
+  description: string | null;
+  start_date: string | null;
+  end_date: string | null;
+  status: string;
+  planning_stage: string | null;
+  currency: string;
+}
+
+@Injectable()
+export class TripsRepository {
+  constructor(@Inject(KYSELY_DB) private readonly db: Kysely<Database>) {}
+
+  async createTrip(ownerId: string, dto: CreateTripDto): Promise<Trip> {
+    return this.db.transaction().execute(async (trx) => {
+      const trip = await trx
+        .insertInto('trips')
+        .values({
+          name: dto.name,
+          description: dto.description ?? null,
+          owner_id: ownerId,
+          start_date: dto.startDate ?? null,
+          end_date: dto.endDate ?? null,
+          status: 'DRAFT',
+          currency: dto.currency ?? 'EUR',
+        })
+        .returningAll()
+        .executeTakeFirstOrThrow();
+
+      await trx
+        .insertInto('trip_settings')
+        .values({
+          trip_id: trip.id,
+          web_research_enabled: DEFAULT_TRIP_SETTINGS.webResearchEnabled,
+          periodic_agent_review_enabled:
+            DEFAULT_TRIP_SETTINGS.periodicAgentReviewEnabled,
+          notification_email_enabled:
+            DEFAULT_TRIP_SETTINGS.notificationEmailEnabled,
+          notification_push_enabled:
+            DEFAULT_TRIP_SETTINGS.notificationPushEnabled,
+        })
+        .execute();
+
+      await trx
+        .insertInto('trip_members')
+        .values({
+          trip_id: trip.id,
+          user_id: ownerId,
+          role: 'OWNER',
+          status: 'ACTIVE',
+          joined_at: new Date().toISOString(),
+        })
+        .execute();
+
+      return toTrip(trip);
+    });
+  }
+
+  async findById(tripId: string): Promise<Trip | undefined> {
+    const row = await this.db
+      .selectFrom('trips')
+      .selectAll()
+      .where('id', '=', tripId)
+      .executeTakeFirst();
+    return row ? toTrip(row) : undefined;
+  }
+
+  async listForUser(userId: string): Promise<Trip[]> {
+    const rows = await this.db
+      .selectFrom('trips')
+      .innerJoin('trip_members', 'trip_members.trip_id', 'trips.id')
+      .selectAll('trips')
+      .where('trip_members.user_id', '=', userId)
+      .where('trip_members.status', '=', 'ACTIVE')
+      .execute();
+    return rows.map(toTrip);
+  }
+
+  async updateWithVersionCheck(
+    tripId: string,
+    expectedVersion: number,
+    patch: Partial<TripUpdateFields>,
+  ): Promise<Trip | undefined> {
+    const row = await this.db
+      .updateTable('trips')
+      .set({
+        ...patch,
+        version: sql`version + 1`,
+        updated_at: new Date().toISOString(),
+      })
+      .where('id', '=', tripId)
+      .where('version', '=', expectedVersion)
+      .returningAll()
+      .executeTakeFirst();
+
+    return row ? toTrip(row) : undefined;
+  }
+
+  async delete(tripId: string): Promise<void> {
+    await this.db.deleteFrom('trips').where('id', '=', tripId).execute();
+  }
+}
diff --git a/backend/libs/trips/src/trips.service.spec.ts b/backend/libs/trips/src/trips.service.spec.ts
new file mode 100644
index 0000000..8f6448b
--- /dev/null
+++ b/backend/libs/trips/src/trips.service.spec.ts
@@ -0,0 +1,33 @@
+import { ConflictException } from '@nestjs/common';
+import { TripsService } from './trips.service';
+
+describe('TripsService.updateTrip', () => {
+  it('throws ConflictException when the repository update matches no row', async () => {
+    const repo = {
+      updateWithVersionCheck: jest.fn().mockResolvedValue(undefined),
+    };
+    const service = new TripsService(repo as never);
+
+    await expect(
+      service.updateTrip('trip-1', { name: 'New name', version: 1 }),
+    ).rejects.toThrow(ConflictException);
+
+    expect(repo.updateWithVersionCheck).toHaveBeenCalledWith(
+      'trip-1',
+      1,
+      expect.objectContaining({ name: 'New name' }),
+    );
+  });
+
+  it('returns the updated trip when the version matches', async () => {
+    const updated = { id: 'trip-1', name: 'New name', version: 2 };
+    const repo = {
+      updateWithVersionCheck: jest.fn().mockResolvedValue(updated),
+    };
+    const service = new TripsService(repo as never);
+
+    await expect(
+      service.updateTrip('trip-1', { name: 'New name', version: 1 }),
+    ).resolves.toEqual(updated);
+  });
+});
diff --git a/backend/libs/trips/src/trips.service.ts b/backend/libs/trips/src/trips.service.ts
new file mode 100644
index 0000000..3d4e1cb
--- /dev/null
+++ b/backend/libs/trips/src/trips.service.ts
@@ -0,0 +1,55 @@
+import {
+  ConflictException,
+  Injectable,
+  NotFoundException,
+} from '@nestjs/common';
+import { TripsRepository } from './trips.repository';
+import type { TripUpdateFields } from './trips.repository';
+import type { CreateTripDto, Trip, UpdateTripDto } from './trip.types';
+
+@Injectable()
+export class TripsService {
+  constructor(private readonly tripsRepository: TripsRepository) {}
+
+  createTrip(ownerId: string, dto: CreateTripDto): Promise<Trip> {
+    return this.tripsRepository.createTrip(ownerId, dto);
+  }
+
+  listTripsForUser(userId: string): Promise<Trip[]> {
+    return this.tripsRepository.listForUser(userId);
+  }
+
+  async getTrip(tripId: string): Promise<Trip> {
+    const trip = await this.tripsRepository.findById(tripId);
+    if (!trip) throw new NotFoundException('Trip not found');
+    return trip;
+  }
+
+  async updateTrip(tripId: string, dto: UpdateTripDto): Promise<Trip> {
+    const patch: Partial<TripUpdateFields> = {};
+    if (dto.name !== undefined) patch.name = dto.name;
+    if (dto.description !== undefined) patch.description = dto.description;
+    if (dto.startDate !== undefined) patch.start_date = dto.startDate;
+    if (dto.endDate !== undefined) patch.end_date = dto.endDate;
+    if (dto.status !== undefined) patch.status = dto.status;
+    if (dto.planningStage !== undefined)
+      patch.planning_stage = dto.planningStage;
+    if (dto.currency !== undefined) patch.currency = dto.currency;
+
+    const updated = await this.tripsRepository.updateWithVersionCheck(
+      tripId,
+      dto.version,
+      patch,
+    );
+    if (!updated) {
+      throw new ConflictException(
+        'Trip was modified by someone else. Reload and retry.',
+      );
+    }
+    return updated;
+  }
+
+  async deleteTrip(tripId: string): Promise<void> {
+    await this.tripsRepository.delete(tripId);
+  }
+}
diff --git a/backend/libs/trips/test/trips.optimistic-locking.integration-spec.ts b/backend/libs/trips/test/trips.optimistic-locking.integration-spec.ts
new file mode 100644
index 0000000..38fe33f
--- /dev/null
+++ b/backend/libs/trips/test/trips.optimistic-locking.integration-spec.ts
@@ -0,0 +1,55 @@
+import { ConflictException } from '@nestjs/common';
+import { Kysely, PostgresDialect } from 'kysely';
+import { Pool } from 'pg';
+import type { Database } from '../../database/src';
+import { TripsRepository } from '../src/trips.repository';
+import { TripsService } from '../src/trips.service';
+import { UsersRepository } from '../../users/src/users.repository';
+
+describe('Trip optimistic locking (integration)', () => {
+  let db: Kysely<Database>;
+  let tripsService: TripsService;
+  let ownerId: string;
+
+  beforeAll(async () => {
+    db = new Kysely<Database>({
+      dialect: new PostgresDialect({
+        pool: new Pool({ connectionString: process.env.DATABASE_URL }),
+      }),
+    });
+    const usersRepository = new UsersRepository(db);
+    const owner = await usersRepository.upsertByExternalSubjectId(
+      'optimistic-locking-test-sub',
+      {
+        email: 'owner@example.test',
+        displayName: 'Owner',
+      },
+    );
+    ownerId = owner.id;
+    tripsService = new TripsService(new TripsRepository(db));
+  });
+
+  afterAll(async () => {
+    await db.destroy();
+  });
+
+  it('rejects a second update that used a stale version', async () => {
+    const trip = await tripsService.createTrip(ownerId, {
+      name: 'Slovenia 2027',
+    });
+    expect(trip.version).toBe(1);
+
+    const firstUpdate = await tripsService.updateTrip(trip.id, {
+      name: 'Slovenia 2027 (v2)',
+      version: 1,
+    });
+    expect(firstUpdate.version).toBe(2);
+
+    await expect(
+      tripsService.updateTrip(trip.id, {
+        name: 'Conflicting concurrent edit',
+        version: 1,
+      }),
+    ).rejects.toThrow(ConflictException);
+  });
+});