diff --git a/src/user/user.service.spec.ts b/src/user/user.service.spec.ts
index 2798c9d..2b560d5 100644
--- a/src/user/user.service.spec.ts
+++ b/src/user/user.service.spec.ts
@@ -1,7 +1,8 @@
 import { Test, TestingModule } from '@nestjs/testing';
 import { UserService } from './user.service';
+import { User } from './interfaces/user.interface';
 
-describe('UsersService', () => {
+describe('UserService', () => {
     let service: UserService;
 
     beforeEach(async () => {
@@ -15,4 +16,129 @@ describe('UsersService', () => {
     it('should be defined', () => {
         expect(service).toBeDefined();
     });
+
+    describe('create()', () => {
+        it('should create a user and return user ID', () => {
+            const user: User = {
+                name: 'Alice',
+                email: 'alice@mail.com',
+                password: '1234',
+            };
+            const id: number = service.create(user);
+            expect(typeof id).toBe('number');
+        });
+
+        it('should create multiple users with unique IDs', () => {
+            const user1: User = {
+                name: 'User1',
+                email: 'u1@mail.com',
+                password: '123',
+            };
+            const user2: User = {
+                name: 'User2',
+                email: 'u2@mail.com',
+                password: '456',
+            };
+            const id1 = service.create(user1);
+            const id2 = service.create(user2);
+            expect(id1).not.toBe(id2);
+        });
+    });
+
+    describe('findAll()', () => {
+        it('should return an array of users', () => {
+            service.create({ name: 'A', email: 'a@mail.com', password: '123' });
+            const users: User[] = service.findAll();
+            expect(Array.isArray(users)).toBe(true);
+            expect(users.length).toBeGreaterThan(0);
+        });
+    });
+
+    describe('findOne()', () => {
+        it('should return the user by ID', () => {
+            const newUser: User = {
+                name: 'Bob',
+                email: 'bob@mail.com',
+                password: 'pass',
+            };
+            const id = service.create(newUser);
+            const user = service.findOne(id);
+            expect(user).toBeDefined();
+            expect(user?.name).toBe('Bob');
+        });
+
+        it('should return undefined for non-existent ID', () => {
+            const result = service.findOne(9999);
+            expect(result).toBeUndefined();
+        });
+    });
+
+    describe('findByUsername()', () => {
+        it('should find a user by name', () => {
+            const user: User = {
+                name: 'Alice',
+                email: 'alice@mail.com',
+                password: '1234',
+            };
+            service.create(user);
+            const result = service.findByUsername('Alice');
+            expect(result).toBeDefined();
+            expect(result?.email).toBe('alice@mail.com');
+        });
+
+        it('should return undefined if username not found', () => {
+            const result = service.findByUsername('NotExist');
+            expect(result).toBeUndefined();
+        });
+    });
+
+    describe('update()', () => {
+        it("should update a user's data", () => {
+            const user: User = {
+                name: 'Old',
+                email: 'old@mail.com',
+                password: '123',
+            };
+            const id = service.create(user);
+
+            const updated = service.update(id, {
+                name: 'New',
+                email: 'new@mail.com',
+                password: '456',
+            });
+
+            expect(updated).toBe(true);
+
+            const result = service.findOne(id);
+            expect(result?.name).toBe('New');
+            expect(result?.email).toBe('new@mail.com');
+        });
+
+        it('should return false when updating non-existent user', () => {
+            const result = service.update(999, {
+                name: 'Ghost',
+            });
+            expect(result).toBe(false);
+        });
+    });
+
+    describe('remove()', () => {
+        it('should delete a user', () => {
+            const user: User = {
+                name: 'Temp',
+                email: 'temp@mail.com',
+                password: 'temp',
+            };
+            const id = service.create(user);
+
+            const deleted = service.remove(id);
+            expect(deleted).toBe(true);
+            expect(service.findOne(id)).toBeUndefined();
+        });
+
+        it('should return false for non-existent user', () => {
+            const deleted = service.remove(12345);
+            expect(deleted).toBe(false);
+        });
+    });
 });
diff --git a/src/user/user.service.ts b/src/user/user.service.ts
index cfdedbe..7da727f 100644
--- a/src/user/user.service.ts
+++ b/src/user/user.service.ts
@@ -15,8 +15,7 @@ export class UserService {
         this.users.set(this.userIDcounter, user);
         return this.userIDcounter;
     }
-    findAll() {
-        // TODO: #26 - get all users from DB
+    findAll(): User[] {
         return Array.from(this.users.values());
     }
     findOne(userID: number): User | undefined {
@@ -28,6 +27,11 @@ export class UserService {
         // parseInt will not work because the defined type is already "number"
         return this.users.get(+userID);
     }
+    findByUsername(username: string): User | undefined {
+        // eslint-disable-next-line @typescript-eslint/no-unnecessary-type-assertion
+        const usersArray = Array.from(this.users.values()) as User[];
+        return usersArray.find((user: User) => user.name === username);
+    }
     update(userID: number, user: UpdateUserDto): boolean {
         // TODO: #26 - update user in DB
         if (this.users.has(+userID)) {