diff --git a/src/application/state/state.service.ts b/src/application/state/state.service.ts
new file mode 100644
index 0000000..dcad708
--- /dev/null
+++ b/src/application/state/state.service.ts
@@ -0,0 +1,10 @@
+import type { State } from "../../domain/state/state.entity.js";
+import type { StateRepository } from "../../domain/state/state.repository.js";
+
+export class StateService {
+  constructor(private readonly stateRepo: StateRepository) {}
+
+  async getAll(): Promise<State[]> {
+    return this.stateRepo.findAll();
+  }
+}
diff --git a/src/application/task/task.service.ts b/src/application/task/task.service.ts
new file mode 100644
index 0000000..d73bea5
--- /dev/null
+++ b/src/application/task/task.service.ts
@@ -0,0 +1,137 @@
+import type { Task, TaskState } from "../../domain/task/task.entity.js";
+import type { TaskRepository } from "../../domain/task/task.repository.js";
+import type { StateRepository } from "../../domain/state/state.repository.js";
+import type { CreateTaskDto, UpdateTaskDto, AddNoteDto } from "../../shared/dto.js";
+import { AppError } from "../../shared/app-error.js";
+
+const VALID_TRANSITIONS: Record<TaskState, TaskState[]> = {
+  new: ["active"],
+  active: ["resolved", "closed"],
+  resolved: ["closed", "active"],
+  closed: [],
+};
+
+export class TaskService {
+  constructor(
+    private readonly taskRepo: TaskRepository,
+    private readonly stateRepo: StateRepository
+  ) {}
+
+  async getAll(): Promise<Task[]> {
+    return this.taskRepo.findAll();
+  }
+
+  async getById(id: string): Promise<Task> {
+    const task = await this.taskRepo.findById(id);
+    if (!task) {
+      throw new AppError(404, `Task with id '${id}' not found`);
+    }
+    return task;
+  }
+
+  async create(dto: CreateTaskDto): Promise<Task> {
+    const now = new Date().toISOString().split("T")[0];
+
+    const newTask: Task = {
+      id: crypto.randomUUID(),
+      title: dto.title,
+      description: dto.description,
+      dueDate: dto.dueDate,
+      completed: false,
+      deletedAt: null,
+      stateHistory: [{ state: "new", date: now }],
+      notes: dto.notes ?? [],
+    };
+
+    return this.taskRepo.create(newTask);
+  }
+
+  async update(id: string, dto: UpdateTaskDto): Promise<Task> {
+    await this.getById(id);
+
+    const updated = await this.taskRepo.update(id, {
+      ...(dto.title !== undefined && { title: dto.title }),
+      ...(dto.description !== undefined && { description: dto.description }),
+      ...(dto.dueDate !== undefined && { dueDate: dto.dueDate }),
+      ...(dto.completed !== undefined && { completed: dto.completed }),
+      ...(dto.notes !== undefined && { notes: dto.notes }),
+    });
+
+    if (!updated) {
+      throw new AppError(500, "Failed to update task");
+    }
+
+    return updated;
+  }
+
+  async delete(id: string): Promise<void> {
+    await this.getById(id);
+    const deleted = await this.taskRepo.delete(id);
+    if (!deleted) {
+      throw new AppError(500, "Failed to delete task");
+    }
+  }
+
+  async transitionState(id: string, newState: TaskState): Promise<Task> {
+    const task = await this.getById(id);
+    const currentState = task.stateHistory[task.stateHistory.length - 1]?.state;
+
+    if (!currentState) {
+      throw new AppError(500, "Task has no state history");
+    }
+
+    const allowed = VALID_TRANSITIONS[currentState];
+    if (!allowed.includes(newState)) {
+      throw new AppError(400, `Cannot transition from '${currentState}' to '${newState}'. Allowed: ${allowed.join(", ") || "none"}`);
+    }
+
+    const validState = await this.stateRepo.findByName(newState);
+    if (!validState) {
+      throw new AppError(400, `Invalid state '${newState}'`);
+    }
+
+    const now = new Date().toISOString().split("T")[0];
+    const updated = await this.taskRepo.update(id, {
+      stateHistory: [...task.stateHistory, { state: newState, date: now }],
+      completed: newState === "closed",
+    });
+
+    if (!updated) {
+      throw new AppError(500, "Failed to transition task state");
+    }
+
+    return updated;
+  }
+
+  async addNote(id: string, dto: AddNoteDto): Promise<Task> {
+    const task = await this.getById(id);
+
+    const updated = await this.taskRepo.update(id, {
+      notes: [...task.notes, dto.note],
+    });
+
+    if (!updated) {
+      throw new AppError(500, "Failed to add note");
+    }
+
+    return updated;
+  }
+
+  async deleteNote(id: string, noteIndex: number): Promise<Task> {
+    const task = await this.getById(id);
+
+    if (noteIndex < 0 || noteIndex >= task.notes.length) {
+      throw new AppError(400, `Invalid note index ${noteIndex}`);
+    }
+
+    const notes = [...task.notes];
+    notes.splice(noteIndex, 1);
+
+    const updated = await this.taskRepo.update(id, { notes });
+    if (!updated) {
+      throw new AppError(500, "Failed to delete note");
+    }
+
+    return updated;
+  }
+}