diff --git a/tests/test_export.py b/tests/test_export.py
new file mode 100644
index 0000000..ef69eaa
--- /dev/null
+++ b/tests/test_export.py
@@ -0,0 +1,52 @@
+"""Tests of the creation and the export of the scores table."""
+
+from pathlib import Path
+
+import pytest
+
+pandas = pytest.importorskip("pandas")
+
+from weather import export  # noqa: E402
+
+
+def test_create_scores_frame_has_the_columns_and_values_of_the_lecture() -> None:
+    frame = export.create_scores_frame()
+
+    assert isinstance(frame, pandas.DataFrame)
+    assert list(frame.columns) == ["students", "scores"]
+    assert frame["students"].to_list() == ["Amy", "James", "Angela"]
+    assert frame["scores"].to_list() == [76, 56, 65]
+
+
+def test_export_frame_round_trips_through_a_file(tmp_path: Path) -> None:
+    frame = export.create_scores_frame()
+    target = tmp_path / "scores.csv"
+
+    export.export_frame(frame, target)
+
+    assert target.is_file()
+    read_back = pandas.read_csv(target, index_col=0)
+    pandas.testing.assert_frame_equal(read_back, frame)
+
+
+def test_export_frame_accepts_a_string_path(tmp_path: Path) -> None:
+    target = tmp_path / "as_text.csv"
+
+    export.export_frame(export.create_scores_frame(), str(target))
+
+    assert target.is_file()
+
+
+def test_export_frame_default_name_writes_only_new_data_csv_in_the_working_dir(
+    tmp_path: Path,
+) -> None:
+    # The autouse fixture in conftest.py made tmp_path the working directory.
+    export.export_frame(export.create_scores_frame())
+
+    assert [item.name for item in tmp_path.iterdir()] == ["new_data.csv"]
+
+
+def test_create_scores_frame_writes_nothing(tmp_path: Path) -> None:
+    export.create_scores_frame()
+
+    assert list(tmp_path.iterdir()) == []
diff --git a/tests/test_main.py b/tests/test_main.py
new file mode 100644
index 0000000..ed505ee
--- /dev/null
+++ b/tests/test_main.py
@@ -0,0 +1,68 @@
+"""Tests of the command line: the printed results and what is written."""
+
+import sys
+from pathlib import Path
+
+import pytest
+
+from weather.__main__ import main
+
+
+def test_main_without_arguments_prints_the_results_of_the_lectures(
+    capsys: pytest.CaptureFixture[str],
+) -> None:
+    pytest.importorskip("pandas")
+
+    assert main([]) == 0
+
+    output = capsys.readouterr().out
+    assert "17.428" in output
+    assert "24" in output
+    assert "Sunny" in output
+    assert "53.6" in output
+    assert "Sunday" in output
+    assert "Angela" in output
+
+
+def test_main_without_the_flag_writes_no_file(tmp_path: Path) -> None:
+    pytest.importorskip("pandas")
+
+    assert main([]) == 0
+
+    assert list(tmp_path.iterdir()) == []
+
+
+def test_main_with_the_export_flag_writes_the_scores_frame(
+    tmp_path: Path, capsys: pytest.CaptureFixture[str]
+) -> None:
+    pandas = pytest.importorskip("pandas")
+    from weather.export import create_scores_frame
+
+    target = tmp_path / "out.csv"
+
+    assert main(["--export", str(target)]) == 0
+
+    assert [item.name for item in tmp_path.iterdir()] == ["out.csv"]
+    read_back = pandas.read_csv(target, index_col=0)
+    pandas.testing.assert_frame_equal(read_back, create_scores_frame())
+    assert str(target) in capsys.readouterr().out
+
+
+def test_main_reports_the_missing_pandas_extra_and_writes_nothing(
+    tmp_path: Path,
+    capsys: pytest.CaptureFixture[str],
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    # None in sys.modules makes "import pandas" raise ImportError; the modules
+    # that import it are removed so that they are imported again.
+    monkeypatch.setitem(sys.modules, "pandas", None)
+    monkeypatch.delitem(sys.modules, "weather.analysis", raising=False)
+    monkeypatch.delitem(sys.modules, "weather.export", raising=False)
+
+    assert main(["--export", str(tmp_path / "out.csv")]) == 1
+
+    captured = capsys.readouterr()
+    assert 'pip install "weather[pandas]"' in captured.err
+    assert "Sunny" in captured.out
+    assert "17.428" not in captured.out
+    assert list(tmp_path.iterdir()) == []