diff --git a/pyproject.toml b/pyproject.toml
index 925a26b..81809d4 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -22,7 +22,7 @@ where = ["src"]
 
 [tool.pytest.ini_options]
 testpaths = ["tests"]
-pythonpath = ["src"]
+pythonpath = ["src", "."]
 
 [tool.ruff]
 line-length = 88
diff --git a/tests/__init__.py b/tests/__init__.py
new file mode 100644
index 0000000..3360580
--- /dev/null
+++ b/tests/__init__.py
@@ -0,0 +1,4 @@
+"""!
+@file __init__.py
+@brief Test package.
+"""
diff --git a/tests/conftest.py b/tests/conftest.py
new file mode 100644
index 0000000..a2748db
--- /dev/null
+++ b/tests/conftest.py
@@ -0,0 +1,25 @@
+"""!
+@file conftest.py
+@brief Shared pytest helpers.
+"""
+
+from collections.abc import Callable, Iterable
+
+import pytest
+
+ScriptInput = Callable[[Iterable[str]], None]
+
+
+@pytest.fixture
+def script_input(monkeypatch: pytest.MonkeyPatch) -> ScriptInput:
+    """!
+    @brief Provide a function that feeds the given answers to `input`.
+    @param monkeypatch pytest's monkeypatch fixture.
+    @return A function taking the answers in the order they are asked.
+    """
+
+    def feed(answers: Iterable[str]) -> None:
+        remaining = iter(answers)
+        monkeypatch.setattr("builtins.input", lambda _prompt: next(remaining))
+
+    return feed
diff --git a/tests/test_machine.py b/tests/test_machine.py
new file mode 100644
index 0000000..2bf7b3f
--- /dev/null
+++ b/tests/test_machine.py
@@ -0,0 +1,107 @@
+"""!
+@file test_machine.py
+@brief Tests for making drinks and the main loop.
+"""
+
+import pytest
+
+from coffee_machine.constants import MENU
+from coffee_machine.main import main, make_coffee
+from tests.conftest import ScriptInput
+
+COINS_FOR_LATTE = ["10", "0", "0", "0"]
+NO_COINS = ["0", "0", "0", "0"]
+FULL_REPORT = "Water: 300ml\nMilk: 200ml\nCoffee: 100g\nMoney: $0.00"
+
+
+def test_make_coffee_uses_up_ingredients(capsys: pytest.CaptureFixture[str]) -> None:
+    """!
+    @brief The drink's ingredients are deducted and the drink is served.
+    """
+    resources = {"water": 300, "milk": 200, "coffee": 100}
+    make_coffee("latte", MENU["latte"]["ingredients"], resources)
+    assert resources == {"water": 100, "milk": 50, "coffee": 76}
+    assert capsys.readouterr().out == "Here is your latte. Enjoy!\n"
+
+
+def test_off_ends_the_machine(script_input: ScriptInput) -> None:
+    """!
+    @brief Typing off returns from the loop.
+    """
+    script_input(["off"])
+    main()
+
+
+def test_report_shows_starting_resources(
+    script_input: ScriptInput, capsys: pytest.CaptureFixture[str]
+) -> None:
+    """!
+    @brief A fresh machine reports full resources and no money.
+    """
+    script_input(["report", "off"])
+    main()
+    assert FULL_REPORT in capsys.readouterr().out
+
+
+def test_unknown_input_is_rejected(
+    script_input: ScriptInput, capsys: pytest.CaptureFixture[str]
+) -> None:
+    """!
+    @brief Anything that is not a drink or command prints a message.
+    """
+    script_input(["tea", "off"])
+    main()
+    assert "Sorry, we do not serve that." in capsys.readouterr().out
+
+
+def test_sale_updates_resources_and_profit(
+    script_input: ScriptInput, capsys: pytest.CaptureFixture[str]
+) -> None:
+    """!
+    @brief A paid drink is served, resources drop and profit rises by the price.
+    """
+    script_input(["latte", *COINS_FOR_LATTE, "report", "off"])
+    main()
+    out = capsys.readouterr().out
+    assert "Here is your latte. Enjoy!" in out
+    assert "Water: 100ml\nMilk: 50ml\nCoffee: 76g\nMoney: $2.50" in out
+
+
+def test_refund_changes_nothing(
+    script_input: ScriptInput, capsys: pytest.CaptureFixture[str]
+) -> None:
+    """!
+    @brief An underpaid order is refunded: no drink, no profit.
+    """
+    script_input(["espresso", *NO_COINS, "report", "off"])
+    main()
+    out = capsys.readouterr().out
+    assert "Money refunded." in out
+    assert "Enjoy" not in out
+    assert FULL_REPORT in out
+
+
+def test_overpayment_adds_only_price_to_profit(
+    script_input: ScriptInput, capsys: pytest.CaptureFixture[str]
+) -> None:
+    """!
+    @brief Change is returned and only the price is counted as profit.
+    """
+    script_input(["espresso", "7", "0", "0", "0", "report", "off"])
+    main()
+    out = capsys.readouterr().out
+    assert "Here is $0.25 in change." in out
+    assert "Money: $1.50" in out
+
+
+def test_order_refused_when_resources_run_out(
+    script_input: ScriptInput, capsys: pytest.CaptureFixture[str]
+) -> None:
+    """!
+    @brief After the water is used up, the next order is refused before payment.
+    """
+    script_input(["latte", *COINS_FOR_LATTE, "latte", "off"])
+    main()
+    out = capsys.readouterr().out
+    assert out.count("Please insert coins.") == 1
+    assert "Sorry there is not enough water." in out
diff --git a/tests/test_resources.py b/tests/test_resources.py
new file mode 100644
index 0000000..e95142d
--- /dev/null
+++ b/tests/test_resources.py
@@ -0,0 +1,52 @@
+"""!
+@file test_resources.py
+@brief Tests for the report and the resource check.
+"""
+
+import pytest
+
+from coffee_machine.constants import INITIAL_RESOURCES, MENU
+from coffee_machine.main import format_money, is_resource_sufficient, report
+
+
+def test_format_money_shows_two_decimals() -> None:
+    """!
+    @brief Cents are shown as dollars with two decimals.
+    """
+    assert format_money(250) == "$2.50"
+    assert format_money(0) == "$0.00"
+
+
+def test_report_prints_resources_and_money(capsys: pytest.CaptureFixture[str]) -> None:
+    """!
+    @brief The report lists each resource with its unit, then the money.
+    """
+    report({"water": 300, "milk": 200, "coffee": 100}, 250)
+    assert capsys.readouterr().out == (
+        "Water: 300ml\nMilk: 200ml\nCoffee: 100g\nMoney: $2.50\n"
+    )
+
+
+def test_is_resource_sufficient_when_enough() -> None:
+    """!
+    @brief An order is accepted when every ingredient is available.
+    """
+    assert is_resource_sufficient(MENU["latte"]["ingredients"], INITIAL_RESOURCES)
+
+
+def test_is_resource_sufficient_when_exactly_enough() -> None:
+    """!
+    @brief Using up an ingredient completely is allowed.
+    """
+    assert is_resource_sufficient({"water": 300}, {"water": 300})
+
+
+def test_is_resource_sufficient_names_missing_resource(
+    capsys: pytest.CaptureFixture[str],
+) -> None:
+    """!
+    @brief A short ingredient is refused and named.
+    """
+    resources = {"water": 300, "milk": 100, "coffee": 100}
+    assert not is_resource_sufficient(MENU["latte"]["ingredients"], resources)
+    assert capsys.readouterr().out == "Sorry there is not enough milk.\n"
diff --git a/tests/test_transactions.py b/tests/test_transactions.py
new file mode 100644
index 0000000..0f57a9e
--- /dev/null
+++ b/tests/test_transactions.py
@@ -0,0 +1,56 @@
+"""!
+@file test_transactions.py
+@brief Tests for coin processing and payment.
+"""
+
+import pytest
+
+from coffee_machine.main import is_transaction_successful, process_coins
+from tests.conftest import ScriptInput
+
+
+def test_process_coins_totals_in_cents(script_input: ScriptInput) -> None:
+    """!
+    @brief Quarters, dimes, nickels and pennies add up to the right total.
+    """
+    script_input(["4", "3", "2", "1"])
+    assert process_coins() == 4 * 25 + 3 * 10 + 2 * 5 + 1
+
+
+def test_process_coins_asks_again_on_invalid_count(
+    script_input: ScriptInput, capsys: pytest.CaptureFixture[str]
+) -> None:
+    """!
+    @brief Text and negative counts are rejected and asked again.
+    """
+    script_input(["abc", "-1", "2", "0", "0", "0"])
+    assert process_coins() == 50
+    assert capsys.readouterr().out.count("Please enter a whole number") == 2
+
+
+def test_transaction_refused_when_too_little(
+    capsys: pytest.CaptureFixture[str],
+) -> None:
+    """!
+    @brief Too little money is refunded with the refund message.
+    """
+    assert not is_transaction_successful(100, 150)
+    assert "Sorry that's not enough money. Money refunded." in capsys.readouterr().out
+
+
+def test_transaction_accepted_with_exact_payment(
+    capsys: pytest.CaptureFixture[str],
+) -> None:
+    """!
+    @brief Exact payment succeeds and gives no change.
+    """
+    assert is_transaction_successful(150, 150)
+    assert "change" not in capsys.readouterr().out
+
+
+def test_transaction_gives_change(capsys: pytest.CaptureFixture[str]) -> None:
+    """!
+    @brief Overpaying succeeds and returns the difference.
+    """
+    assert is_transaction_successful(175, 150)
+    assert capsys.readouterr().out == "Here is $0.25 in change.\n"