diff --git a/maternal-app/maternal-app-backend/src/modules/auth/auth.module.ts b/maternal-app/maternal-app-backend/src/modules/auth/auth.module.ts
index b68a8b2..f5345a6 100644
--- a/maternal-app/maternal-app-backend/src/modules/auth/auth.module.ts
+++ b/maternal-app/maternal-app-backend/src/modules/auth/auth.module.ts
@@ -22,6 +22,7 @@ import {
   Family,
   FamilyMember,
 } from '../../database/entities';
+import { InviteCode, InviteCodeUse } from '../invite-codes/invite-codes.entity';
 
 @Module({
   imports: [
@@ -33,6 +34,8 @@ import {
       Family,
       FamilyMember,
       WebAuthnCredential,
+      InviteCode,
+      InviteCodeUse,
     ]),
     PassportModule,
     CommonModule,
diff --git a/maternal-app/maternal-app-backend/src/modules/auth/auth.service.ts b/maternal-app/maternal-app-backend/src/modules/auth/auth.service.ts
index de7a14d..de480da 100644
--- a/maternal-app/maternal-app-backend/src/modules/auth/auth.service.ts
+++ b/maternal-app/maternal-app-backend/src/modules/auth/auth.service.ts
@@ -20,6 +20,7 @@ import {
   AuditAction,
   EntityType,
 } from '../../database/entities';
+import { InviteCode, InviteCodeUse } from '../invite-codes/invite-codes.entity';
 import { RegisterDto } from './dto/register.dto';
 import { LoginDto } from './dto/login.dto';
 import { RefreshTokenDto } from './dto/refresh-token.dto';
@@ -43,12 +44,49 @@ export class AuthService {
     private familyRepository: Repository<Family>,
     @InjectRepository(FamilyMember)
     private familyMemberRepository: Repository<FamilyMember>,
+    @InjectRepository(InviteCode)
+    private inviteCodeRepository: Repository<InviteCode>,
+    @InjectRepository(InviteCodeUse)
+    private inviteCodeUseRepository: Repository<InviteCodeUse>,
     private jwtService: JwtService,
     private configService: ConfigService,
     private auditService: AuditService,
   ) {}
 
   async register(registerDto: RegisterDto): Promise<AuthResponse> {
+    // Check registration mode and validate invite code if required
+    const registrationMode = this.configService.get<string>('REGISTRATION_MODE', 'invite_only');
+    const requireInviteCode = registrationMode === 'invite_only';
+
+    let validatedInviteCode: InviteCode | null = null;
+
+    if (requireInviteCode) {
+      if (!registerDto.inviteCode) {
+        throw new BadRequestException(
+          'An invite code is required to register. Please contact an administrator for an invite code.',
+        );
+      }
+
+      // Validate the invite code
+      validatedInviteCode = await this.inviteCodeRepository.findOne({
+        where: { code: registerDto.inviteCode, isActive: true },
+      });
+
+      if (!validatedInviteCode) {
+        throw new BadRequestException('Invalid or inactive invite code');
+      }
+
+      // Check if the invite code has expired
+      if (validatedInviteCode.expiresAt && new Date(validatedInviteCode.expiresAt) < new Date()) {
+        throw new BadRequestException('This invite code has expired');
+      }
+
+      // Check if the invite code has reached its maximum number of uses
+      if (validatedInviteCode.maxUses && validatedInviteCode.uses >= validatedInviteCode.maxUses) {
+        throw new BadRequestException('This invite code has reached its maximum number of uses');
+      }
+    }
+
     // Check if user already exists
     const existingUser = await this.userRepository.findOne({
       where: { email: registerDto.email },
@@ -99,6 +137,28 @@ export class AuthService {
 
     const savedUser = await this.userRepository.save(user);
 
+    // Record invite code usage if applicable
+    if (validatedInviteCode) {
+      // Increment the usage counter
+      validatedInviteCode.uses += 1;
+      await this.inviteCodeRepository.save(validatedInviteCode);
+
+      // Record the invite code usage
+      const inviteCodeUse = this.inviteCodeUseRepository.create({
+        inviteCodeId: validatedInviteCode.id,
+        usedBy: savedUser.id,
+        userEmail: savedUser.email,
+        userIp: null, // Could be passed from the request if needed
+        userAgent: null, // Could be passed from the request if needed
+      });
+
+      await this.inviteCodeUseRepository.save(inviteCodeUse);
+
+      this.logger.log(
+        `User ${savedUser.email} registered using invite code ${validatedInviteCode.code}`,
+      );
+    }
+
     // Create default family for user
     const family = this.familyRepository.create({
       name: `${registerDto.name}'s Family`,
diff --git a/maternal-app/maternal-app-backend/src/modules/auth/dto/register.dto.ts b/maternal-app/maternal-app-backend/src/modules/auth/dto/register.dto.ts
index 0b35cad..dae2e5e 100644
--- a/maternal-app/maternal-app-backend/src/modules/auth/dto/register.dto.ts
+++ b/maternal-app/maternal-app-backend/src/modules/auth/dto/register.dto.ts
@@ -57,6 +57,10 @@ export class RegisterDto {
   @IsEmail()
   parentalEmail?: string; // Parent/guardian email for users 13-17
 
+  @IsOptional()
+  @IsString()
+  inviteCode?: string; // Invite code (required if REGISTRATION_MODE=invite_only)
+
   @IsObject()
   deviceInfo: DeviceInfoDto;
 }