diff --git a/rosdistro_reviewer/git_lines.py b/rosdistro_reviewer/git_lines.py
new file mode 100644
index 0000000..1580b62
--- /dev/null
+++ b/rosdistro_reviewer/git_lines.py
@@ -0,0 +1,96 @@
+# Copyright 2024 Open Source Robotics Foundation, Inc.
+# Licensed under the Apache License, Version 2.0
+
+from typing import Dict
+from typing import Iterable
+from typing import List
+from typing import Mapping
+from typing import Optional
+from typing import Sequence
+
+from git import Repo
+import unidiff
+
+
+def _rangeify(sequence: Iterable[int]) -> Iterable[range]:
+    chunk_last = None
+    chunk_start = None
+
+    for item in sequence:
+        if chunk_last != item - 1:
+            if chunk_start is not None:
+                yield range(chunk_start, chunk_last + 1)
+            chunk_start = item
+        chunk_last = item
+
+    if chunk_start is not None and chunk_last is not None:
+        yield range(chunk_start, chunk_last + 1)
+
+
+def get_added_lines(
+    path,
+    *,
+    target_ref: Optional[str] = None,
+    head_ref: Optional[str] = None,
+    paths=None,
+) -> Optional[Mapping[str, Sequence[range]]]:
+    """
+    Determine what lines were added between two git repository states.
+
+    :param path: The path to the repository root
+    :param target_ref: The git ref to base the diff from
+    :param head_ref: The git ref where the changes have been made
+    :param paths: Relative paths under the repository to limit results to
+
+    :returns: Mapping of relative file paths to sequences of line number
+        ranges, or None if no changes were detected
+    """
+    repo = Repo(path)
+
+    if head_ref is not None:
+        head = repo.commit(head_ref)
+    else:
+        head = None
+
+    if target_ref is not None:
+        target = repo.commit(target_ref)
+    elif head is not None:
+        target = head.parents[0]
+    else:
+        target = repo.head.commit
+
+    if head is not None:
+        for base in repo.merge_base(target, head):
+            if base is not None:
+                break
+        else:
+            raise RuntimeError(
+                f"No merge base found between '{target_ref}' and '{head_ref}'")
+    else:
+        base = target
+
+    diffs = base.diff(head, paths, True)
+
+    lines: Dict[str, List[int]] = {}
+    for diff in diffs:
+        if not diff.b_path:
+            continue
+        patch = f"""--- {diff.a_path if diff.a_path else '/dev/null'}
++++ {diff.b_path}
+{diff.diff.decode()}"""
+        patchset = unidiff.PatchSet(patch)
+        for file in patchset:
+            for hunk in file:
+                for line in hunk:
+                    if line.line_type != unidiff.LINE_TYPE_ADDED:
+                        continue
+                    lines.setdefault(file.path, [])
+                    lines[file.path].append(line.target_line_no)
+
+    if not lines:
+        return None
+
+    return {
+        path: list(_rangeify(sorted(lines.get(path, ()))))
+        for path in (paths if paths is not None else lines.keys())
+    }
diff --git a/setup.cfg b/setup.cfg
index 986dc58..1aa83a1 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -20,6 +20,8 @@ long_description_content_type = text/markdown
 [options]
 python_requires = >=3.6
 install_requires =
+    GitPython
+    unidiff
 packages = find:
 zip_safe = true
 
diff --git a/test/spell_check.words b/test/spell_check.words
index 4fba70c..ddc1c4f 100644
--- a/test/spell_check.words
+++ b/test/spell_check.words
@@ -1,13 +1,19 @@
+addfinalizer
 apache
+diffs
 https
 iterdir
 linter
+mktemp
 mypy
+patchset
 pathlib
 pycqa
 pytest
+rangeify
 returncode
 rosdistro
 scspell
 setuptools
 thomas
+unidiff
diff --git a/test/test_git_lines.py b/test/test_git_lines.py
new file mode 100644
index 0000000..62c97f6
--- /dev/null
+++ b/test/test_git_lines.py
@@ -0,0 +1,118 @@
+# Copyright 2024 Open Source Robotics Foundation, Inc.
+# Licensed under the Apache License, Version 2.0
+
+from git import Head
+from git import Repo
+import pytest
+from rosdistro_reviewer.git_lines import get_added_lines
+
+
+@pytest.fixture(scope='session')
+def git_repo(
+    request: pytest.FixtureRequest,
+    tmp_path_factory: pytest.TempPathFactory,
+) -> Repo:
+    repo_dir = tmp_path_factory.mktemp('git_repo')
+    repo = Repo.init(repo_dir)
+    request.addfinalizer(repo.close)
+
+    repo.index.commit('Initial commit')
+
+    base = repo.create_head('base')
+    base.checkout()
+    lines_txt = repo_dir / 'lines.txt'
+    with open(lines_txt, 'w') as f:
+        f.write('\n'.join(['a', 'b', 'c', 'd', 'e', 'B', 'E', '']))
+    repo.index.add((lines_txt,))
+    repo.index.commit('Add lines.txt')
+
+    repo.head.reference = Head(repo, 'refs/heads/orphan')  # type: ignore
+    repo.index.commit('Orphaned commit')
+
+    repo.create_head('lines2', 'base').checkout()
+    lines2_txt = repo_dir / 'lines2.txt'
+    with open(lines2_txt, 'w') as f:
+        f.write('\n'.join(['1', '2']))
+    repo.index.add((lines2_txt,))
+    repo.index.remove(str(lines_txt), working_tree=True)
+    repo.index.commit('Add lines2.txt, remove lines.txt')
+
+    repo.create_head('less_c', 'base').checkout()
+    with open(lines_txt, 'w') as f:
+        f.write('\n'.join(['a', 'b', 'd', 'e', 'B', 'C', 'E', '']))
+    repo.index.add((lines_txt,))
+    repo.index.commit("Remove 'c' from lines.txt")
+
+    repo.create_head('less_c_d', 'less_c').checkout()
+    with open(lines_txt, 'w') as f:
+        f.write('\n'.join(['a', 'b', 'e', 'B', 'C', 'D', 'E', '']))
+    repo.index.add((lines_txt,))
+    repo.index.commit("Remove 'd' from lines.txt")
+
+    repo.create_head('less_a', 'base').checkout()
+    with open(lines_txt, 'w') as f:
+        f.write('\n'.join(['b', 'c', 'd', 'e', 'A', 'B', 'E', '']))
+    repo.index.add((lines_txt,))
+    repo.index.commit("Remove 'a' from lines.txt")
+
+    target = repo.create_head('merge_c_d_to_a', 'less_a').checkout()
+    other = repo.heads['less_c_d']
+    repo.index.merge_tree(other.commit)
+    with open(lines_txt, 'w') as f:
+        f.write('\n'.join(['b', 'e', 'A', 'B', 'C', 'D', 'E', '']))
+    repo.index.add((lines_txt,))
+    repo.index.commit(
+        "Merge branch 'less_c_d' into merge_c_d_to_a",
+        parent_commits=[target.commit, other.commit])  # type: ignore
+
+    with open(lines_txt, 'a') as f:
+        f.write('X\n')
+
+    return repo
+
+
+def test_added_lines(git_repo: Repo) -> None:
+    # Check uncommitted
+    lines = get_added_lines(git_repo.working_dir)
+    assert lines == {'lines.txt': [range(8, 9)]}
+
+    # Check path targeting
+    lines = get_added_lines(git_repo.working_dir, paths=['lines.txt'])
+    assert lines == {'lines.txt': [range(8, 9)]}
+
+    # Check path targeting with no match
+    lines = get_added_lines(git_repo.working_dir, paths=['foo.txt'])
+    assert lines is None
+
+    # Check explicit head
+    lines = get_added_lines(git_repo.working_dir, head_ref='less_a')
+    assert lines == {'lines.txt': [range(5, 6)]}
+
+    # Check explicit target with no head (including uncommitted)
+    lines = get_added_lines(git_repo.working_dir, target_ref='less_c')
+    assert lines == {'lines.txt': [range(3, 4), range(6, 7), range(8, 9)]}
+
+    # Check explicit head and target
+    lines = get_added_lines(git_repo.working_dir, target_ref='less_c',
+                            head_ref='less_c_d')
+    assert lines == {'lines.txt': [range(6, 7)]}
+
+    # Check explicit head and target with multiple commits
+    lines = get_added_lines(git_repo.working_dir, target_ref='base',
+                            head_ref='less_c_d')
+    assert lines == {'lines.txt': [range(5, 7)]}
+
+    # Check merge base behavior
+    lines = get_added_lines(git_repo.working_dir, target_ref='less_a',
+                            head_ref='less_c_d')
+    assert lines == {'lines.txt': [range(5, 7)]}
+
+    # Check file being added
+    lines = get_added_lines(git_repo.working_dir, target_ref='base',
+                            head_ref='lines2')
+    assert lines == {'lines2.txt': [range(1, 3)]}
+
+    # Check failure to find merge base
+    with pytest.raises(RuntimeError):
+        get_added_lines(git_repo.working_dir, target_ref='orphan',
+                        head_ref='less_a')