diff --git a/tests/e2e/test_password_manager_window.py b/tests/e2e/test_password_manager_window.py
new file mode 100644
index 0000000..2660ae5
--- /dev/null
+++ b/tests/e2e/test_password_manager_window.py
@@ -0,0 +1,65 @@
+import tkinter as tk
+from collections.abc import Iterator
+from pathlib import Path
+
+import pytest
+
+from password_manager import constants
+from password_manager.application.use_cases import (
+    FindPasswordEntry,
+    GeneratePassword,
+    SavePasswordEntry,
+)
+from password_manager.infrastructure.persistence.json_password_repository import (
+    JsonPasswordRepository,
+)
+from password_manager.presentation.features.password_manager import (
+    password_manager_view as view_module,
+)
+from password_manager.presentation.features.password_manager import (
+    password_manager_view_model as view_model_module,
+)
+
+
+@pytest.fixture
+def root() -> Iterator[tk.Tk]:
+    try:
+        window = tk.Tk()
+    except tk.TclError:
+        pytest.skip("no display available")
+    window.withdraw()
+    yield window
+    window.destroy()
+
+
+def test_save_then_search_through_window(
+    root: tk.Tk, tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    shown: list[str] = []
+    monkeypatch.setattr(view_module.messagebox, "askokcancel", lambda **_: True)
+    monkeypatch.setattr(
+        view_module.messagebox, "showinfo", lambda **kw: shown.append(kw["message"])
+    )
+    repository = JsonPasswordRepository(tmp_path / "data.json")
+    view = view_module.PasswordManagerView(
+        root,
+        view_model_module.PasswordManagerViewModel(
+            SavePasswordEntry(repository),
+            FindPasswordEntry(repository),
+            GeneratePassword(),
+        ),
+    )
+    view.website_entry.insert(0, "example.com")
+    view.generate_password()
+    password = view.password_entry.get()
+    assert root.clipboard_get() == password
+
+    view.save()
+
+    assert view.website_entry.get() == ""
+    assert view.password_entry.get() == ""
+    view.website_entry.insert(0, "example.com")
+    view.find_password()
+    assert shown[-1] == constants.MESSAGE_DETAILS.format(
+        email=constants.DEFAULT_EMAIL, password=password
+    )
diff --git a/tests/integration/test_json_password_repository.py b/tests/integration/test_json_password_repository.py
new file mode 100644
index 0000000..be0e384
--- /dev/null
+++ b/tests/integration/test_json_password_repository.py
@@ -0,0 +1,111 @@
+import json
+from pathlib import Path
+
+import pytest
+
+from password_manager.domain.errors import (
+    DataFileNotFoundError,
+    EntryNotFoundError,
+    InvalidDataFileError,
+)
+from password_manager.domain.password_entry import PasswordEntry
+from password_manager.infrastructure.persistence.json_password_repository import (
+    JsonPasswordRepository,
+)
+
+ENTRY = PasswordEntry("example.com", "me@x.org", "pw1")
+
+
+def test_add_writes_nested_json_and_reads_back(tmp_path: Path) -> None:
+    path = tmp_path / "data.json"
+    repository = JsonPasswordRepository(path)
+
+    repository.add(ENTRY)
+
+    assert json.loads(path.read_text("utf-8")) == {
+        "example.com": {"email": "me@x.org", "password": "pw1"}
+    }
+    assert repository.find("example.com") == ENTRY
+
+
+def test_add_uses_indent_of_four(tmp_path: Path) -> None:
+    path = tmp_path / "data.json"
+    JsonPasswordRepository(path).add(ENTRY)
+
+    assert '\n    "example.com"' in path.read_text("utf-8")
+
+
+def test_add_keeps_existing_entries(tmp_path: Path) -> None:
+    repository = JsonPasswordRepository(tmp_path / "data.json")
+    other = PasswordEntry("other.org", "o@x.org", "pw2")
+    repository.add(ENTRY)
+
+    repository.add(other)
+
+    assert repository.find("example.com") == ENTRY
+    assert repository.find("other.org") == other
+
+
+def test_add_creates_missing_file(tmp_path: Path) -> None:
+    path = tmp_path / "data.json"
+    assert not path.exists()
+
+    JsonPasswordRepository(path).add(ENTRY)
+
+    assert path.exists()
+
+
+def test_add_rewrites_empty_file(tmp_path: Path) -> None:
+    path = tmp_path / "data.json"
+    path.write_text("", encoding="utf-8")
+
+    JsonPasswordRepository(path).add(ENTRY)
+
+    assert JsonPasswordRepository(path).find("example.com") == ENTRY
+
+
+def test_add_rewrites_invalid_json(tmp_path: Path) -> None:
+    path = tmp_path / "data.json"
+    path.write_text("{not json", encoding="utf-8")
+
+    JsonPasswordRepository(path).add(ENTRY)
+
+    assert JsonPasswordRepository(path).find("example.com") == ENTRY
+
+
+def test_add_refuses_json_that_is_not_an_object(tmp_path: Path) -> None:
+    path = tmp_path / "data.json"
+    path.write_text("[1, 2]", encoding="utf-8")
+
+    with pytest.raises(InvalidDataFileError):
+        JsonPasswordRepository(path).add(ENTRY)
+    assert path.read_text("utf-8") == "[1, 2]"
+
+
+def test_find_raises_when_file_missing(tmp_path: Path) -> None:
+    with pytest.raises(DataFileNotFoundError):
+        JsonPasswordRepository(tmp_path / "data.json").find("example.com")
+
+
+def test_find_raises_when_website_absent(tmp_path: Path) -> None:
+    repository = JsonPasswordRepository(tmp_path / "data.json")
+    repository.add(ENTRY)
+
+    with pytest.raises(EntryNotFoundError):
+        repository.find("nope.com")
+
+
+def test_find_raises_not_found_on_empty_file(tmp_path: Path) -> None:
+    path = tmp_path / "data.json"
+    path.write_text("", encoding="utf-8")
+
+    with pytest.raises(EntryNotFoundError):
+        JsonPasswordRepository(path).find("example.com")
+
+
+def test_find_raises_when_entry_lacks_keys(tmp_path: Path) -> None:
+    path = tmp_path / "data.json"
+    path.write_text('{"example.com": {"email": "e"}}', encoding="utf-8")
+
+    with pytest.raises(InvalidDataFileError):
+        JsonPasswordRepository(path).find("example.com")
diff --git a/tests/unit/application/test_use_cases.py b/tests/unit/application/test_use_cases.py
new file mode 100644
index 0000000..eb0e923
--- /dev/null
+++ b/tests/unit/application/test_use_cases.py
@@ -0,0 +1,68 @@
+import random
+
+import pytest
+from conftest import FakePasswordRepository
+
+from password_manager.application.use_cases import (
+    FindPasswordEntry,
+    GeneratePassword,
+    SavePasswordEntry,
+)
+from password_manager.domain.errors import (
+    DataFileNotFoundError,
+    EmptyFieldError,
+    EntryNotFoundError,
+)
+from password_manager.domain.password_entry import PasswordEntry
+
+
+def test_save_stores_entry(repository: FakePasswordRepository) -> None:
+    SavePasswordEntry(repository).execute("example.com", "me@x.org", "pw")
+
+    assert repository.entries["example.com"] == PasswordEntry(
+        "example.com", "me@x.org", "pw"
+    )
+
+
+@pytest.mark.parametrize(
+    ("website", "email", "password"),
+    [("", "e", "p"), ("w", "", "p"), ("w", "e", ""), ("  ", "e", "p")],
+)
+def test_save_rejects_empty_field(
+    repository: FakePasswordRepository, website: str, email: str, password: str
+) -> None:
+    with pytest.raises(EmptyFieldError):
+        SavePasswordEntry(repository).execute(website, email, password)
+    assert repository.entries == {}
+
+
+def test_find_returns_entry(repository: FakePasswordRepository) -> None:
+    SavePasswordEntry(repository).execute("example.com", "me@x.org", "pw")
+
+    entry = FindPasswordEntry(repository).execute("example.com")
+
+    assert entry.email == "me@x.org"
+
+
+def test_find_raises_when_website_unknown(
+    repository: FakePasswordRepository,
+) -> None:
+    with pytest.raises(EntryNotFoundError):
+        FindPasswordEntry(repository).execute("nope.com")
+
+
+def test_find_raises_when_no_data_file() -> None:
+    with pytest.raises(DataFileNotFoundError):
+        FindPasswordEntry(FakePasswordRepository(has_file=False)).execute("a.com")
+
+
+def test_find_rejects_empty_website(repository: FakePasswordRepository) -> None:
+    with pytest.raises(EmptyFieldError):
+        FindPasswordEntry(repository).execute("")
+
+
+def test_generate_uses_given_random_source() -> None:
+    first = GeneratePassword(random.Random(1)).execute()
+    second = GeneratePassword(random.Random(1)).execute()
+
+    assert first == second
diff --git a/tests/unit/conftest.py b/tests/unit/conftest.py
new file mode 100644
index 0000000..901c0f5
--- /dev/null
+++ b/tests/unit/conftest.py
@@ -0,0 +1,30 @@
+"""Shared fakes of the ports for the unit tests."""
+
+import pytest
+
+from password_manager.domain.errors import DataFileNotFoundError, EntryNotFoundError
+from password_manager.domain.password_entry import PasswordEntry
+
+
+class FakePasswordRepository:
+    """In-memory PasswordRepository; `has_file` False mimics a missing file."""
+
+    def __init__(self, has_file: bool = True) -> None:
+        self.entries: dict[str, PasswordEntry] = {}
+        self.has_file = has_file
+
+    def add(self, entry: PasswordEntry) -> None:
+        self.entries[entry.website] = entry
+        self.has_file = True
+
+    def find(self, website: str) -> PasswordEntry:
+        if not self.has_file:
+            raise DataFileNotFoundError
+        if website not in self.entries:
+            raise EntryNotFoundError(website)
+        return self.entries[website]
+
+
+@pytest.fixture
+def repository() -> FakePasswordRepository:
+    return FakePasswordRepository()
diff --git a/tests/unit/domain/test_password_generator.py b/tests/unit/domain/test_password_generator.py
new file mode 100644
index 0000000..4bb3a86
--- /dev/null
+++ b/tests/unit/domain/test_password_generator.py
@@ -0,0 +1,35 @@
+import random
+
+from password_manager import constants
+from password_manager.domain.password_generator import generate_password
+
+
+def _count(password: str, alphabet: str) -> int:
+    return sum(1 for char in password if char in alphabet)
+
+
+def test_generate_password_length_within_bounds() -> None:
+    low = constants.MIN_LETTERS + constants.MIN_SYMBOLS + constants.MIN_DIGITS
+    high = constants.MAX_LETTERS + constants.MAX_SYMBOLS + constants.MAX_DIGITS
+    for seed in range(50):
+        assert low <= len(generate_password(random.Random(seed))) <= high
+
+
+def test_generate_password_uses_all_three_classes_within_bounds() -> None:
+    for seed in range(50):
+        password = generate_password(random.Random(seed))
+        letters = _count(password, constants.LETTERS)
+        symbols = _count(password, constants.SYMBOLS)
+        digits = _count(password, constants.DIGITS)
+        assert constants.MIN_LETTERS <= letters <= constants.MAX_LETTERS
+        assert constants.MIN_SYMBOLS <= symbols <= constants.MAX_SYMBOLS
+        assert constants.MIN_DIGITS <= digits <= constants.MAX_DIGITS
+        assert letters + symbols + digits == len(password)
+
+
+def test_generate_password_is_reproducible_with_same_seed() -> None:
+    assert generate_password(random.Random(7)) == generate_password(random.Random(7))
+
+
+def test_generate_password_default_source_works() -> None:
+    assert generate_password()
diff --git a/tests/unit/presentation/test_password_manager_view_model.py b/tests/unit/presentation/test_password_manager_view_model.py
new file mode 100644
index 0000000..6c5cf79
--- /dev/null
+++ b/tests/unit/presentation/test_password_manager_view_model.py
@@ -0,0 +1,121 @@
+import random
+
+from conftest import FakePasswordRepository
+
+from password_manager import constants
+from password_manager.application.use_cases import (
+    FindPasswordEntry,
+    GeneratePassword,
+    SavePasswordEntry,
+)
+from password_manager.presentation.features.password_manager import (
+    password_manager_view_model as view_model_module,
+)
+
+
+class FakeUi:
+    """Records clipboard writes and questions, answers with a fixed choice."""
+
+    def __init__(self, answer: bool = True) -> None:
+        self.answer = answer
+        self.clipboard: list[str] = []
+        self.questions: list[str] = []
+
+    def copy(self, text: str) -> None:
+        self.clipboard.append(text)
+
+    def confirm(self, message: str) -> bool:
+        self.questions.append(message)
+        return self.answer
+
+
+def _make(
+    repository: FakePasswordRepository, ui: FakeUi
+) -> view_model_module.PasswordManagerViewModel:
+    return view_model_module.PasswordManagerViewModel(
+        SavePasswordEntry(repository),
+        FindPasswordEntry(repository),
+        GeneratePassword(random.Random(3)),
+        ui.copy,
+        ui.confirm,
+    )
+
+
+def test_generate_password_copies_to_clipboard(
+    repository: FakePasswordRepository,
+) -> None:
+    ui = FakeUi()
+
+    password = _make(repository, ui).generate_password()
+
+    assert ui.clipboard == [password]
+
+
+def test_save_stores_and_clears_fields_after_confirmation(
+    repository: FakePasswordRepository,
+) -> None:
+    ui = FakeUi(answer=True)
+
+    result = _make(repository, ui).save("a.com", "me@x.org", "pw")
+
+    assert result is not None
+    assert not result.is_error
+    assert result.clear_fields
+    assert "a.com" in repository.entries
+    assert "me@x.org" in ui.questions[0]
+
+
+def test_save_does_nothing_when_user_cancels(
+    repository: FakePasswordRepository,
+) -> None:
+    result = _make(repository, FakeUi(answer=False)).save("a.com", "e", "pw")
+
+    assert result is None
+    assert repository.entries == {}
+
+
+def test_save_refuses_empty_field_without_asking(
+    repository: FakePasswordRepository,
+) -> None:
+    ui = FakeUi()
+
+    result = _make(repository, ui).save("", "e", "pw")
+
+    assert result is not None
+    assert result.is_error
+    assert result.message == constants.MESSAGE_EMPTY_FIELDS
+    assert not result.clear_fields
+    assert ui.questions == []
+
+
+def test_find_password_found(repository: FakePasswordRepository) -> None:
+    model = _make(repository, FakeUi())
+    model.save("a.com", "me@x.org", "pw")
+
+    result = model.find_password("a.com")
+
+    assert not result.is_error
+    assert result.message == constants.MESSAGE_DETAILS.format(
+        email="me@x.org", password="pw"
+    )
+
+
+def test_find_password_website_not_present(
+    repository: FakePasswordRepository,
+) -> None:
+    result = _make(repository, FakeUi()).find_password("b.com")
+
+    assert result.is_error
+    assert result.message == constants.MESSAGE_NO_DETAILS.format(website="b.com")
+
+
+def test_find_password_no_data_file() -> None:
+    model = _make(FakePasswordRepository(has_file=False), FakeUi())
+
+    assert model.find_password("a.com").message == constants.MESSAGE_NO_DATA_FILE
+
+
+def test_find_password_empty_website(repository: FakePasswordRepository) -> None:
+    result = _make(repository, FakeUi()).find_password(" ")
+
+    assert result.message == constants.MESSAGE_EMPTY_FIELDS