diff --git a/src/modules/authentication/authentication.module.ts b/src/modules/authentication/authentication.module.ts
index 8a9401d..dc0c776 100644
--- a/src/modules/authentication/authentication.module.ts
+++ b/src/modules/authentication/authentication.module.ts
@@ -16,7 +16,7 @@ import { MessageBrokerPublisher } from '@mobicoop/message-broker-module';
 import { UsernameRepository } from './infrastructure/username.repository';
 import { UsernameMapper } from './username.mapper';
 import { AddUsernameGrpcController } from './interface/grpc-controllers/add-username.grpc.controller';
-import { AddUsernameService } from './core/application/commands/add-usernames/add-username.service';
+import { AddUsernameService } from './core/application/commands/add-username/add-username.service';
 
 const grpcControllers = [
   CreateAuthenticationGrpcController,
diff --git a/src/modules/authentication/core/application/commands/add-usernames/add-username.command.ts b/src/modules/authentication/core/application/commands/add-username/add-username.command.ts
similarity index 100%
rename from src/modules/authentication/core/application/commands/add-usernames/add-username.command.ts
rename to src/modules/authentication/core/application/commands/add-username/add-username.command.ts
diff --git a/src/modules/authentication/core/application/commands/add-usernames/add-username.service.ts b/src/modules/authentication/core/application/commands/add-username/add-username.service.ts
similarity index 100%
rename from src/modules/authentication/core/application/commands/add-usernames/add-username.service.ts
rename to src/modules/authentication/core/application/commands/add-username/add-username.service.ts
diff --git a/src/modules/authentication/interface/grpc-controllers/add-username.grpc.controller.ts b/src/modules/authentication/interface/grpc-controllers/add-username.grpc.controller.ts
index a3d51c6..bc41a4e 100644
--- a/src/modules/authentication/interface/grpc-controllers/add-username.grpc.controller.ts
+++ b/src/modules/authentication/interface/grpc-controllers/add-username.grpc.controller.ts
@@ -9,7 +9,7 @@ import { CommandBus } from '@nestjs/cqrs';
 import { GrpcMethod, RpcException } from '@nestjs/microservices';
 import { UsernameAlreadyExistsException } from '@modules/authentication/core/domain/authentication.errors';
 import { AddUsernameRequestDto } from './dtos/add-username.request.dto';
-import { AddUsernameCommand } from '@modules/authentication/core/application/commands/add-usernames/add-username.command';
+import { AddUsernameCommand } from '@modules/authentication/core/application/commands/add-username/add-username.command';
 
 @UsePipes(
   new RpcValidationPipe({
diff --git a/src/modules/authentication/tests/unit/core/add-username.service.spec.ts b/src/modules/authentication/tests/unit/core/add-username.service.spec.ts
new file mode 100644
index 0000000..8e9411a
--- /dev/null
+++ b/src/modules/authentication/tests/unit/core/add-username.service.spec.ts
@@ -0,0 +1,106 @@
+import {
+  AggregateID,
+  ConflictException,
+  UniqueConstraintException,
+} from '@mobicoop/ddd-library';
+import {
+  AUTHENTICATION_REPOSITORY,
+  USERNAME_REPOSITORY,
+} from '@modules/authentication/authentication.di-tokens';
+import { AddUsernameCommand } from '@modules/authentication/core/application/commands/add-username/add-username.command';
+import { AddUsernameService } from '@modules/authentication/core/application/commands/add-username/add-username.service';
+import { UsernameAlreadyExistsException } from '@modules/authentication/core/domain/authentication.errors';
+import { UsernameEntity } from '@modules/authentication/core/domain/username.entity';
+import { Type } from '@modules/authentication/core/domain/username.types';
+import { AddUsernameRequestDto } from '@modules/authentication/interface/grpc-controllers/dtos/add-username.request.dto';
+import { Test, TestingModule } from '@nestjs/testing';
+
+const addUsernameRequest: AddUsernameRequestDto = {
+  userId: '165192d4-398a-4469-a16b-98c02cc6f531',
+  name: 'john.doe@email.com',
+  type: Type.EMAIL,
+};
+
+const mockAuthenticationRepository = {
+  findOneById: jest.fn().mockImplementationOnce(() => ({})),
+};
+
+const mockUsernameRepository = {
+  insert: jest
+    .fn()
+    .mockImplementationOnce(() => ({}))
+    .mockImplementationOnce(() => {
+      throw new ConflictException('already exists');
+    })
+    .mockImplementationOnce(() => {
+      throw new UniqueConstraintException('username');
+    })
+    .mockImplementationOnce(() => {
+      throw new Error();
+    }),
+};
+
+describe('Add Username Service', () => {
+  let addUsernameService: AddUsernameService;
+
+  beforeAll(async () => {
+    const module: TestingModule = await Test.createTestingModule({
+      providers: [
+        {
+          provide: AUTHENTICATION_REPOSITORY,
+          useValue: mockAuthenticationRepository,
+        },
+        {
+          provide: USERNAME_REPOSITORY,
+          useValue: mockUsernameRepository,
+        },
+        AddUsernameService,
+      ],
+    }).compile();
+
+    addUsernameService = module.get<AddUsernameService>(AddUsernameService);
+  });
+
+  it('should be defined', () => {
+    expect(addUsernameService).toBeDefined();
+  });
+
+  describe('execution', () => {
+    const addUsernameCommand = new AddUsernameCommand({
+      userId: addUsernameRequest.userId,
+      username: {
+        name: addUsernameRequest.name,
+        type: addUsernameRequest.type,
+      },
+    });
+    UsernameEntity.create = jest.fn().mockReturnValue({
+      id: 'john.doe@email.com',
+      getProps: jest.fn().mockImplementation(() => ({
+        userId: '165192d4-398a-4469-a16b-98c02cc6f531',
+        name: 'john.doe@email.com',
+        type: Type.EMAIL,
+      })),
+    });
+    it('should add a new username', async () => {
+      const result: AggregateID = await addUsernameService.execute(
+        addUsernameCommand,
+      );
+      expect(result).toBe('john.doe@email.com');
+    });
+    it('should throw a dedicated exception if username already exists for this type', async () => {
+      await expect(
+        addUsernameService.execute(addUsernameCommand),
+      ).rejects.toBeInstanceOf(UsernameAlreadyExistsException);
+    });
+    it('should throw a dedicated exception if username already exists for this name', async () => {
+      await expect(
+        addUsernameService.execute(addUsernameCommand),
+      ).rejects.toBeInstanceOf(UsernameAlreadyExistsException);
+    });
+    it('should throw an error if something bad happens', async () => {
+      await expect(
+        addUsernameService.execute(addUsernameCommand),
+      ).rejects.toBeInstanceOf(Error);
+    });
+  });
+});
diff --git a/src/modules/authentication/tests/unit/core/create-authentication.service.spec.ts b/src/modules/authentication/tests/unit/core/create-authentication.service.spec.ts
index b4b1d25..b4df3d5 100644
--- a/src/modules/authentication/tests/unit/core/create-authentication.service.spec.ts
+++ b/src/modules/authentication/tests/unit/core/create-authentication.service.spec.ts
@@ -44,7 +44,7 @@ const mockAuthenticationRepository = {
     }),
 };
 
-describe('create-authentication.service', () => {
+describe('Create Authentication Service', () => {
   let createAuthenticationService: CreateAuthenticationService;
 
   beforeAll(async () => {
@@ -68,63 +68,39 @@ describe('create-authentication.service', () => {
   });
 
   describe('execution', () => {
-    const createAdCommand = new CreateAuthenticationCommand(
+    const createAuthenticationCommand = new CreateAuthenticationCommand(
       createAuthenticationRequest,
     );
+    AuthenticationEntity.create = jest.fn().mockReturnValue({
+      id: '165192d4-398a-4469-a16b-98c02cc6f531',
+      getProps: jest.fn().mockImplementation(() => ({
+        userId: '165192d4-398a-4469-a16b-98c02cc6f531',
+      })),
+    });
     it('should create a new authentication', async () => {
-      AuthenticationEntity.create = jest.fn().mockReturnValue({
-        id: '165192d4-398a-4469-a16b-98c02cc6f531',
-        getProps: jest.fn().mockImplementation(() => ({
-          userId: '165192d4-398a-4469-a16b-98c02cc6f531',
-        })),
-      });
       const result: AggregateID = await createAuthenticationService.execute(
-        createAdCommand,
+        createAuthenticationCommand,
       );
       expect(result).toBe('165192d4-398a-4469-a16b-98c02cc6f531');
     });
     it('should throw a dedicated exception if Authentication already exists', async () => {
-      AuthenticationEntity.create = jest.fn().mockReturnValue({
-        id: '165192d4-398a-4469-a16b-98c02cc6f531',
-        getProps: jest.fn().mockImplementation(() => ({
-          userId: '165192d4-398a-4469-a16b-98c02cc6f531',
-        })),
-      });
       await expect(
-        createAuthenticationService.execute(createAdCommand),
+        createAuthenticationService.execute(createAuthenticationCommand),
       ).rejects.toBeInstanceOf(AuthenticationAlreadyExistsException);
     });
     it('should throw a dedicated exception if uuid already exists', async () => {
-      AuthenticationEntity.create = jest.fn().mockReturnValue({
-        id: '165192d4-398a-4469-a16b-98c02cc6f531',
-        getProps: jest.fn().mockImplementation(() => ({
-          userId: '165192d4-398a-4469-a16b-98c02cc6f531',
-        })),
-      });
       await expect(
-        createAuthenticationService.execute(createAdCommand),
+        createAuthenticationService.execute(createAuthenticationCommand),
       ).rejects.toBeInstanceOf(AuthenticationAlreadyExistsException);
     });
     it('should throw a dedicated exception if username already exists', async () => {
-      AuthenticationEntity.create = jest.fn().mockReturnValue({
-        id: '165192d4-398a-4469-a16b-98c02cc6f531',
-        getProps: jest.fn().mockImplementation(() => ({
-          userId: '165192d4-398a-4469-a16b-98c02cc6f531',
-        })),
-      });
       await expect(
-        createAuthenticationService.execute(createAdCommand),
+        createAuthenticationService.execute(createAuthenticationCommand),
       ).rejects.toBeInstanceOf(UsernameAlreadyExistsException);
     });
     it('should throw an error if something bad happens', async () => {
-      AuthenticationEntity.create = jest.fn().mockReturnValue({
-        id: '165192d4-398a-4469-a16b-98c02cc6f531',
-        getProps: jest.fn().mockImplementation(() => ({
-          userId: '165192d4-398a-4469-a16b-98c02cc6f531',
-        })),
-      });
       await expect(
-        createAuthenticationService.execute(createAdCommand),
+        createAuthenticationService.execute(createAuthenticationCommand),
       ).rejects.toBeInstanceOf(Error);
     });
   });