diff --git a/backend/service/services.py b/backend/service/services.py
new file mode 100644
index 0000000..fdbddd2
--- /dev/null
+++ b/backend/service/services.py
@@ -0,0 +1,16 @@
+from backend.model.tribe import Tribe
+from backend.repository.abstract_repository import AbstractRepository
+from backend.repository.tribe_sqlite_repository import TribeRepositoryError
+
+
+class TribeExists(Exception):
+    pass
+
+
+def add_tribe(tribe: Tribe, tribe_repo: AbstractRepository, conn):
+    try:
+        tribe_repo.add(tribe)
+    except TribeRepositoryError:
+        raise TribeExists(f"The tribe {tribe.name} already exists")
+    conn.commit()
+    return tribe
diff --git a/tests/model/fakes.py b/tests/model/fakes.py
index 6ffa738..0ce9369 100644
--- a/tests/model/fakes.py
+++ b/tests/model/fakes.py
@@ -91,35 +91,3 @@ def build_student(
             Student(name=faker.name(), tribe=tribe) for _ in range(students_per_tribe)
         ]
     return students
-
-
-class FakeTribeRepository(AbstractRepository):
-    def __init__(self, tribes):
-        self._tribes = {t.name: t for t in tribes}
-
-    def add(self, tribe):
-        if tribe.name not in self._tribes.keys():
-            self._tribes[tribe.name] = tribe
-        raise KeyError(f"{tribe} already exists")
-
-    def update(self, name, tribe):
-        try:
-            self._tribes.pop(name)
-            self._tribes[tribe.name] = tribe
-        except KeyError:
-            raise KeyError(f"The tribe {tribe} does not exists")
-
-    def list(self):
-        return list(self._tribes.values())
-
-    def get(self, name):
-        try:
-            return self._tribes[name]
-        except KeyError:
-            raise KeyError(f"The tribe {tribe} does not exists")
-
-    def delete(self, name):
-        try:
-            self._tribes.pop(name)
-        except KeyError:
-            raise KeyError(f"The tribe {tribe} does not exists")
diff --git a/tests/unit/test_service.py b/tests/unit/test_service.py
new file mode 100644
index 0000000..df19ad4
--- /dev/null
+++ b/tests/unit/test_service.py
@@ -0,0 +1,108 @@
+import pytest
+
+from backend.model.student import Student
+from backend.model.tribe import Tribe
+from backend.repository.abstract_repository import AbstractRepository
+from backend.repository.student_sqlite_repository import StudentRepositoryError
+from backend.repository.tribe_sqlite_repository import TribeRepositoryError
+from backend.service import services
+from backend.service.services import TribeExists
+from tests.model.fakes import build_tribes
+
+
+class FakeTribeRepository(AbstractRepository):
+    def __init__(self, tribes: list[Tribe] = []) -> None:
+        self._tribes = {t.name: t for t in tribes}
+
+    def add(self, tribe: Tribe) -> None:
+        if tribe.name not in self._tribes.keys():
+            self._tribes[tribe.name] = tribe
+        else:
+            raise TribeRepositoryError(f"{tribe} already exists")
+
+    def update(self, name: str, tribe: Tribe) -> None:
+        try:
+            self._tribes.pop(name)
+            self._tribes[tribe.name] = tribe
+        except KeyError:
+            raise TribeRepositoryError(f"The tribe {tribe} does not exists")
+
+    def list(self) -> list[Tribe]:
+        return list(self._tribes.values())
+
+    def get(self, name: str) -> Tribe:
+        try:
+            return self._tribes[name]
+        except KeyError:
+            raise TribeRepositoryError(f"The tribe {tribe} does not exists")
+
+    def delete(self, name: str) -> None:
+        try:
+            self._tribes.pop(name)
+        except KeyError:
+            raise KeyError(f"The tribe {tribe} does not exists")
+
+
+class FakeStudentRepository(AbstractRepository):
+    def __init__(self, students: list[Student] = []) -> None:
+        self._students = {s.id: s for s in students}
+
+    def add(self, student: Student) -> None:
+        if student.id not in self._students.keys():
+            self._students[student.id] = student
+        raise StudentRepositoryError(f"{student} already exists")
+
+    def update(self, id: str, student: Student) -> None:
+        try:
+            self._students.pop(id)
+            self._students[student.id] = student
+        except KeyError:
+            raise StudentRepositoryError(f"The student {student} does not exists")
+
+    def list(self) -> list[Student]:
+        return list(self._students.values())
+
+    def get(self, id: str) -> Student:
+        try:
+            return self._students[id]
+        except KeyError:
+            raise KeyError(f"The student {student} does not exists")
+
+    def delete(self, id: str) -> None:
+        try:
+            self._students.pop(id)
+        except KeyError:
+            raise StudentRepositoryError(f"The student {student} does not exists")
+
+
+class FakeConn:
+    committed = False
+
+    def commit(self):
+        self.committed = True
+
+    def reset_commit(self):
+        self.committed = False
+
+
+def test_add_tribe():
+    tribe_repo = FakeTribeRepository()
+    tribe = build_tribes(1)[0]
+    conn = FakeConn()
+    services.add_tribe(tribe, tribe_repo, conn)
+
+    assert conn.committed == True
+    assert tribe_repo.list() == [tribe]
+
+
+def test_add_tribe_fail_exists():
+    tribe_repo = FakeTribeRepository()
+    tribe = build_tribes(1)[0]
+    conn = FakeConn()
+    services.add_tribe(tribe, tribe_repo, conn)
+    conn.reset_commit()
+
+    with pytest.raises(TribeExists):
+        services.add_tribe(tribe, tribe_repo, conn)
+
+    assert conn.committed == False