diff --git a/src/qtxterm/about_dialog.py b/src/qtxterm/about_dialog.py
new file mode 100644
index 0000000..3e6ac47
--- /dev/null
+++ b/src/qtxterm/about_dialog.py
@@ -0,0 +1,73 @@
+"""A small About box: which build is running, and a way to copy that."""
+
+from __future__ import annotations
+
+from PySide6.QtCore import QSize, Qt
+from PySide6.QtGui import QGuiApplication, QIcon
+from PySide6.QtWidgets import (
+    QDialog,
+    QDialogButtonBox,
+    QHBoxLayout,
+    QLabel,
+    QPushButton,
+    QVBoxLayout,
+    QWidget,
+)
+
+from qtxterm.branding import LOGO_PATH
+from qtxterm.version_info import version_string
+
+LOGO_SIZE = 64
+
+COPIED_TEXT = "Copied"
+
+
+class AboutDialog(QDialog):
+    """Shows the running build's version, commit and source location."""
+
+    def __init__(self, parent: QWidget | None = None) -> None:
+        super().__init__(parent)
+        self.setWindowTitle("About qtxterm")
+
+        self.details = version_string()
+
+        layout = QHBoxLayout(self)
+
+        self.logo = QLabel(self)
+        if LOGO_PATH.is_file():
+            self.logo.setPixmap(
+                QIcon(str(LOGO_PATH)).pixmap(QSize(LOGO_SIZE, LOGO_SIZE))
+            )
+        self.logo.setAlignment(Qt.AlignmentFlag.AlignTop)
+        layout.addWidget(self.logo)
+
+        right = QVBoxLayout()
+        layout.addLayout(right)
+
+        self.text = QLabel(self.details, self)
+        # Selectable so the version can be dragged out even without the button,
+        # and monospaced so the aligned labels in version_lines() stay aligned.
+        self.text.setTextInteractionFlags(
+            Qt.TextInteractionFlag.TextSelectableByMouse
+            | Qt.TextInteractionFlag.TextSelectableByKeyboard
+        )
+        self.text.setStyleSheet("font-family: monospace;")
+        right.addWidget(self.text)
+
+        buttons = QDialogButtonBox(QDialogButtonBox.StandardButton.Close, parent=self)
+        self.copy_button = QPushButton("Copy", self)
+        self.copy_button.setDefault(True)
+        buttons.addButton(self.copy_button, QDialogButtonBox.ButtonRole.ActionRole)
+        self.copy_button.clicked.connect(self.copy_details)
+        buttons.rejected.connect(self.reject)
+        right.addWidget(buttons)
+
+    def copy_details(self) -> None:
+        """Put the version block on the clipboard, ready to paste into an issue."""
+        clipboard = QGuiApplication.clipboard()
+        if clipboard is None:
+            return
+        clipboard.setText(self.details)
+        # Confirm in place. A dialog raising another dialog to say "done" is
+        # more interruption than a copy button deserves.
+        self.copy_button.setText(COPIED_TEXT)
diff --git a/src/qtxterm/main_window.py b/src/qtxterm/main_window.py
index 7b1a677..fcfa542 100644
--- a/src/qtxterm/main_window.py
+++ b/src/qtxterm/main_window.py
@@ -4,6 +4,7 @@
 from PySide6.QtGui import QAction
 from PySide6.QtWidgets import QApplication, QDockWidget, QMainWindow, QStyle
 
+from qtxterm.about_dialog import AboutDialog
 from qtxterm.appearance import AppearanceStore
 from qtxterm.branding import app_icon
 from qtxterm.cron import CronStore
@@ -182,10 +183,19 @@ def _build_help_menu(self) -> None:
             self.style().standardIcon(QStyle.StandardPixmap.SP_MessageBoxQuestion)
         )
         self._usage_action.triggered.connect(self.show_usage)
+        self._help_menu.addSeparator()
+        self._about_action = self._help_menu.addAction("About qtxterm")
+        self._about_action.setIcon(
+            self.style().standardIcon(QStyle.StandardPixmap.SP_MessageBoxInformation)
+        )
+        self._about_action.triggered.connect(self.show_about)
 
     def show_usage(self) -> None:
         HelpDialog(self).exec()
 
+    def show_about(self) -> None:
+        AboutDialog(self).exec()
+
     def _build_file_menu(self) -> None:
         # QMenu.addMenu() parents the submenu in C++, but without a Python-side
         # reference kept alive too, PySide6 can garbage-collect the wrapper (and
diff --git a/src/qtxterm/version_info.py b/src/qtxterm/version_info.py
new file mode 100644
index 0000000..691f9c0
--- /dev/null
+++ b/src/qtxterm/version_info.py
@@ -0,0 +1,152 @@
+"""Where the running code came from, for the About dialog.
+
+The packaged version alone cannot answer "which build am I looking at?" -
+`pyproject.toml` carries a static version that only moves on a release, so
+every commit between two releases reports the same string. An editable
+install makes that worse: the code is read live from a checkout, so the
+version is fixed while the code underneath it changes with every branch
+switch.
+
+Git knows the answer and is right there in a checkout, so ask it, and fall
+back to the packaged metadata when it is not - an installed wheel has no
+`.git`, and a user machine may have no `git` at all.
+"""
+
+from __future__ import annotations
+
+import subprocess
+from dataclasses import dataclass
+from datetime import UTC, datetime
+from functools import cache
+from importlib.metadata import PackageNotFoundError, version
+from pathlib import Path
+
+PACKAGE_DIR = Path(__file__).resolve().parent
+
+# src/qtxterm -> src -> repo root. Absent from an installed wheel, which is
+# exactly the signal that this is not a development build.
+REPO_ROOT = PACKAGE_DIR.parent.parent
+
+# Git is normally instant, but it takes locks and can block on a busy repo.
+# The About dialog is not worth hanging the UI for, so cap the wait and
+# degrade to the packaged version instead.
+GIT_TIMEOUT_SECONDS = 2.0
+
+UNKNOWN_VERSION = "unknown"
+
+
+@dataclass(frozen=True)
+class BuildInfo:
+    """Identity of the running build, however much of it could be determined."""
+
+    version: str
+    describe: str | None = None
+    commit_datetime: datetime | None = None
+    source: Path | None = None
+    installed_datetime: datetime | None = None
+
+    @property
+    def is_development(self) -> bool:
+        """Whether this is running from a checkout rather than an installed copy."""
+        return self.describe is not None
+
+
+def _run_git(*args: str) -> str | None:
+    """Run a git command in the repo, or return None if git cannot answer.
+
+    Covers the three ways this fails in the wild: no git on PATH
+    (FileNotFoundError), no repository to read (non-zero exit), and a repo
+    that does not answer promptly (TimeoutExpired).
+    """
+    if not (REPO_ROOT / ".git").exists():
+        return None
+    try:
+        result = subprocess.run(
+            ["git", *args],
+            cwd=REPO_ROOT,
+            capture_output=True,
+            text=True,
+            timeout=GIT_TIMEOUT_SECONDS,
+            check=False,
+        )
+    except (OSError, subprocess.SubprocessError):
+        return None
+    if result.returncode != 0:
+        return None
+    return result.stdout.strip() or None
+
+
+def packaged_version() -> str:
+    """The version recorded at install time, or a placeholder if unreadable."""
+    try:
+        return version("qtxterm")
+    except PackageNotFoundError:
+        return UNKNOWN_VERSION
+
+
+def _commit_datetime() -> datetime | None:
+    raw = _run_git("log", "-1", "--format=%cI")
+    if raw is None:
+        return None
+    try:
+        return datetime.fromisoformat(raw)
+    except ValueError:
+        return None
+
+
+def _installed_datetime() -> datetime | None:
+    """When the package landed on disk.
+
+    The package directory's mtime is written when the wheel is unpacked, so
+    for an installed copy it dates the install. It is meaningless for an
+    editable install, where the directory is the checkout - hence only used
+    when git found nothing.
+    """
+    try:
+        stamp = PACKAGE_DIR.stat().st_mtime
+    except OSError:
+        return None
+    # Read as UTC then converted to local, so the result is timezone-aware
+    # and prints an offset like the git commit date does.
+    return datetime.fromtimestamp(stamp, tz=UTC).astimezone()
+
+
+@cache
+def build_info() -> BuildInfo:
+    """Identify the running build. Cached - the answer cannot change mid-run."""
+    describe = _run_git("describe", "--always", "--dirty", "--tags")
+    if describe is None:
+        return BuildInfo(
+            version=packaged_version(),
+            installed_datetime=_installed_datetime(),
+        )
+    return BuildInfo(
+        version=packaged_version(),
+        describe=describe,
+        commit_datetime=_commit_datetime(),
+        source=REPO_ROOT,
+    )
+
+
+def _format(moment: datetime) -> str:
+    return moment.strftime("%Y-%m-%d %H:%M:%S %z").strip()
+
+
+def version_lines(info: BuildInfo | None = None) -> list[str]:
+    """The About dialog's text, one line per fact that could be determined."""
+    info = info if info is not None else build_info()
+    lines = [f"qtxterm {info.version}"]
+    if info.describe:
+        lines.append(info.describe)
+    if info.commit_datetime:
+        lines.append(f"commit    {_format(info.commit_datetime)}")
+    if info.installed_datetime:
+        lines.append(f"installed {_format(info.installed_datetime)}")
+    if info.source:
+        lines.append(f"source    {info.source}")
+    return lines
+
+
+def version_string(info: BuildInfo | None = None) -> str:
+    """The About dialog's text as one block, also what Copy puts on the clipboard."""
+    return "\n".join(version_lines(info))
diff --git a/tests/test_about_dialog.py b/tests/test_about_dialog.py
new file mode 100644
index 0000000..30cb037
--- /dev/null
+++ b/tests/test_about_dialog.py
@@ -0,0 +1,39 @@
+"""The About dialog shows the build and copies it."""
+
+from __future__ import annotations
+
+from PySide6.QtGui import QGuiApplication
+
+from qtxterm.about_dialog import COPIED_TEXT, AboutDialog
+from qtxterm.version_info import version_string
+
+
+def test_dialog_shows_the_version_block(qtbot) -> None:
+    dialog = AboutDialog()
+    qtbot.addWidget(dialog)
+
+    assert dialog.text.text() == version_string()
+    assert "qtxterm" in dialog.text.text()
+
+
+def test_copy_puts_the_details_on_the_clipboard(qtbot) -> None:
+    dialog = AboutDialog()
+    qtbot.addWidget(dialog)
+
+    dialog.copy_details()
+
+    assert QGuiApplication.clipboard().text() == dialog.details
+    assert dialog.copy_button.text() == COPIED_TEXT
+
+
+def test_dialog_survives_a_missing_logo(qtbot, monkeypatch, tmp_path) -> None:
+    """Assets can be absent from a broken install; the build info still matters."""
+    import qtxterm.about_dialog as about_dialog
+
+    monkeypatch.setattr(about_dialog, "LOGO_PATH", tmp_path / "gone.svg")
+
+    dialog = AboutDialog()
+    qtbot.addWidget(dialog)
+
+    assert dialog.logo.pixmap().isNull()
+    assert dialog.text.text()
diff --git a/tests/test_main_window.py b/tests/test_main_window.py
index befb1cf..12b5c23 100644
--- a/tests/test_main_window.py
+++ b/tests/test_main_window.py
@@ -164,6 +164,46 @@ def exec(self) -> int:
     window.close()
 
 
+def test_help_menu_about_action_opens_the_dialog(
+    qtbot, monkeypatch, tmp_path: Path
+) -> None:
+    """exec() is monkeypatched - a real modal would block the test run."""
+    import qtxterm.main_window as main_window
+
+    opened = []
+
+    class FakeAboutDialog:
+        def __init__(self, parent=None) -> None:
+            opened.append(parent)
+
+        def exec(self) -> int:
+            return 0
+
+    monkeypatch.setattr(main_window, "AboutDialog", FakeAboutDialog)
+
+    window = MainWindow(settings=make_settings(tmp_path))
+    qtbot.addWidget(window)
+    window.show()
+
+    assert window._about_action.text() == "About qtxterm"
+    assert not window._about_action.icon().isNull()
+
+    window._about_action.trigger()
+
+    assert opened == [window]
+
+    window.close()
+
+
+def test_help_menu_lists_usage_then_about(qtbot, tmp_path: Path) -> None:
+    window = MainWindow(settings=make_settings(tmp_path))
+    qtbot.addWidget(window)
+
+    entries = [a.text() for a in window._help_menu.actions() if not a.isSeparator()]
+
+    assert entries == ["Usage", "About qtxterm"]
+
+
 def test_menu_bar_order(qtbot, tmp_path: Path) -> None:
     window = MainWindow(settings=make_settings(tmp_path))
     qtbot.addWidget(window)
diff --git a/tests/test_version_info.py b/tests/test_version_info.py
new file mode 100644
index 0000000..be4451e
--- /dev/null
+++ b/tests/test_version_info.py
@@ -0,0 +1,109 @@
+"""Build identification, including the paths where git cannot answer."""
+
+from __future__ import annotations
+
+import subprocess
+from datetime import UTC, datetime
+
+import pytest
+
+from qtxterm import version_info
+from qtxterm.version_info import BuildInfo, version_lines, version_string
+
+
+@pytest.fixture(autouse=True)
+def _clear_cache():
+    """build_info() is cached for the process; tests need it recomputed."""
+    version_info.build_info.cache_clear()
+    yield
+    version_info.build_info.cache_clear()
+
+
+def test_development_build_reports_git_description() -> None:
+    """The repo under test is a checkout, so git should answer."""
+    info = version_info.build_info()
+
+    assert info.is_development
+    assert info.describe
+    assert info.source == version_info.REPO_ROOT
+    assert info.commit_datetime is not None
+
+
+def test_missing_git_falls_back_to_packaged_metadata(monkeypatch) -> None:
+    """An installed wheel has no .git, and a user machine may have no git."""
+    monkeypatch.setattr(version_info, "_run_git", lambda *args: None)
+
+    info = version_info.build_info()
+
+    assert not info.is_development
+    assert info.describe is None
+    assert info.version == version_info.packaged_version()
+    assert info.installed_datetime is not None
+
+
+def test_git_timeout_is_not_fatal(monkeypatch) -> None:
+    """A slow repo must degrade, not hang the About dialog."""
+
+    def _hang(*args, **kwargs):
+        raise subprocess.TimeoutExpired(cmd="git", timeout=2.0)
+
+    monkeypatch.setattr(subprocess, "run", _hang)
+
+    assert version_info._run_git("describe") is None
+
+
+def test_missing_git_binary_is_not_fatal(monkeypatch) -> None:
+    def _absent(*args, **kwargs):
+        raise FileNotFoundError("git")
+
+    monkeypatch.setattr(subprocess, "run", _absent)
+
+    assert version_info._run_git("describe") is None
+
+
+def test_non_zero_exit_returns_none(monkeypatch) -> None:
+    """Outside a repository git exits non-zero rather than raising."""
+
+    def _fail(*args, **kwargs):
+        return subprocess.CompletedProcess(args=["git"], returncode=128, stdout="")
+
+    monkeypatch.setattr(subprocess, "run", _fail)
+
+    assert version_info._run_git("describe") is None
+
+
+def test_version_lines_for_a_development_build() -> None:
+    info = BuildInfo(
+        version="1.0.0",
+        describe="v1.0.0-21-gd4bd495",
+        commit_datetime=datetime(2026, 9, 20, 21, 41, 22, tzinfo=UTC),
+        source=version_info.REPO_ROOT,
+    )
+
+    lines = version_lines(info)
+
+    assert lines[0] == "qtxterm 1.0.0"
+    assert lines[1] == "v1.0.0-21-gd4bd495"
+    assert any(line.startswith("commit") for line in lines)
+    assert any(line.startswith("source") for line in lines)
+    assert not any(line.startswith("installed") for line in lines)
+
+
+def test_version_lines_for_an_installed_build() -> None:
+    """No commit or source line when there is no checkout to report."""
+    info = BuildInfo(
+        version="1.0.0",
+        installed_datetime=datetime(2026, 9, 20, 15, 5, tzinfo=UTC),
+    )
+
+    lines = version_lines(info)
+
+    assert lines[0] == "qtxterm 1.0.0"
+    assert any(line.startswith("installed") for line in lines)
+    assert not any(line.startswith("commit") for line in lines)
+
+
+def test_version_string_joins_the_lines() -> None:
+    info = BuildInfo(version="1.0.0", describe="abc1234")
+
+    assert version_string(info) == "qtxterm 1.0.0\nabc1234"