diff --git a/README.md b/README.md
index 6c0ed77..35dd221 100644
--- a/README.md
+++ b/README.md
@@ -41,9 +41,6 @@ Missing Features
 The following features are still missing from this version of the
 program.
 
-* Deal with cases when the local branch to which we're committing is
-  behind its remote tracking branch. Normally we'd want the local branch
-  to be fast-forwarded to match the remote before doing our commit.
 * Update the reflog after doing the commit.
 * Add the ability to specify a commit message.
   (This should allow a token to substitute the current HEAD commit at
@@ -66,6 +63,24 @@ The second method will allow you to use `git` options before the
 `commit-filetree` command, such as `-C`, `--git-dir`, `--work-tree`,
 and so on.
 
+#### Fast-forwarding to Tracking Branch Head
+
+If the release branch to which you're committing has a tracking branch,
+the local branch will first be fast-forwarded to the head of that
+tracking branch before the commit is made. This is almost invariably
+the desired behaviour: if someone else has released new versions on
+the release branch you want to continue that history rather than
+diverging. (I.e., you want the `fetch`/`git-commit-filetree`/`push`
+sequence to Just Work without having to do any other manipulation of
+the release branch.)
+
+If your release branch has diverged from its tracking branch,
+`git-commit-filetree` will currently print an error and refuse to
+commit, and you will need to manually resolve the divergence. If you
+wish to be able to commit on a divergent local branch, you can modify
+the script not to generate this error. If there's sufficient demand, a
+command-line option could be added to toggle this behaviour.
+
 ### Usage with Windows
 
 The standard Git installation for Windows includes the Bash shell, and
@@ -86,6 +101,11 @@ systems. However, you should be able to use other TAP test harnesses
 instead. If you have any difficulty, please feel free to contact the
 authors for help.
 
+As a side note, this hacked-together testing framework has probably
+been taken about as far as it reasonably can. If much more functionality
+needs to be added (to the program or the test framework), the framework
+probably wants a rewrite in a better language like Python.
+
 
 Authors and History
 -------------------
diff --git a/bin/git-commit-filetree b/bin/git-commit-filetree
index 9812836..030dcce 100755
--- a/bin/git-commit-filetree
+++ b/bin/git-commit-filetree
@@ -3,7 +3,7 @@
 #  git-commit-filetree - commit an arbitrary tree of files
 #                        to a branch in a git repository.
 #
-#  Version:  cjsnjr.2019-03-23.0
+#  Version:  cjsnjr.2019-03-28.0
 #
 #  Please find the latest version of this (with tests and copying rights) at:
 #    https://github.com/cynic-net/git-commit-filetree
@@ -29,6 +29,28 @@ err() {
     exit $exitcode
 }
 
+fast_forward_to_tracking() {
+    declare -g branch ref tracking
+    #   If local ahead of tracking; carry on.
+    git merge-base --is-ancestor "$tracking" "$ref" && return 0
+    #   If local can fast-foward to tracking; do it.
+    git merge-base --is-ancestor "$ref" "$tracking" && {
+        #   Remember, update-ref must be given a full ref name; just `mybr`
+        #   will create or update that name at the top of the "refs tree,"
+        #   not `refs/heads/mybr`. To help catch this kind of error, we use
+        #   the third argument (required current branch value) where short
+        #   names such as `mybr` are expanded to `refs/heads/mybr`.
+        git update-ref "$ref" "$tracking" "$branch" \
+            -m 'commit-filetree: fast-forward'
+        return 0
+    }
+    #   Local diverged.
+    err 3 "Branch $branch has diverged from tracking $tracking." \
+        $'\n''You must fix this manually before using git-commit-filetree.'
+        #   We should probably extend this explanation with suggestions
+        #   on how to fix the problem.
+}
+
 branch="$1"; shift || true
 path="$1"; shift || true
 [ -z "$path" -o -n "$1" ] \
@@ -43,6 +65,12 @@ git diff-index --quiet HEAD \
 ref=refs/heads/$(echo $branch | sed -e 's,^refs/heads/,,')
 git show-ref -q --verify $ref || err 128 "Invalid ref: $ref"
 
+#   Find and fast-forward the tracking ref, if we have one
+tracking=$(\
+    git rev-parse --symbolic-full-name  "$branch@{upstream}" 2>/dev/null \
+    || true)
+[[ $tracking ]] && fast_forward_to_tracking
+
 source_sha=$(git show --quiet --pretty='format:%h')
 
 # Switch to an index separate from repo working copy.
diff --git a/t/40-commit.t b/t/40-commit.t
index 4f4547c..c1d184e 100644
--- a/t/40-commit.t
+++ b/t/40-commit.t
@@ -2,7 +2,7 @@
 . t/test-lib.sh
 set -e -o 'pipefail'
 
-echo "1..9"
+echo "1..11"
 
 branch=testbr
 repo=tmp/test/repo
@@ -83,12 +83,12 @@ start_test 'Commit data correct'
 make_test_repo
 $git branch $branch
 assert_branch '737b0f439051 refs/heads/master' master
-assert_branch '737b0f439051 refs/heads/testbr'
+assert_branch "737b0f439051 refs/heads/$branch"
 
 echo bar > $files/one
 echo bar > $files/subdir/two
 $git commit-filetree $branch $files
-assert_branch '003e5987f385 refs/heads/testbr'
+assert_branch "003e5987f385 refs/heads/$branch"
 
 end_test
 
@@ -102,7 +102,7 @@ $git branch $branch
 echo bar > $files/one
 echo bar > $files/subdir/two
 $git commit-filetree refs/heads/$branch $files
-assert_branch '003e5987f385 refs/heads/testbr'
+assert_branch "003e5987f385 refs/heads/$branch"
 
 end_test
 
@@ -116,7 +116,7 @@ $git branch $branch
 echo bar > $files/one
 echo bar > $files/subdir/two
 (cd $repo && ../../../bin/git-commit-filetree $branch ../files)
-assert_branch '003e5987f385 refs/heads/testbr'
+assert_branch "003e5987f385 refs/heads/$branch"
 
 end_test
 
@@ -130,7 +130,7 @@ echo bar > $files/subdir/two
 $git commit-filetree $branch $files
 $git commit-filetree $branch $files
 $git commit-filetree $branch $files
-assert_branch '003e5987f385 refs/heads/testbr'
+assert_branch "003e5987f385 refs/heads/$branch"
 end_test
 
 ##### 9
@@ -146,3 +146,80 @@ test_equal \
 737b0f4 testbr@{1}: branch: Created from master' \
     "$($git reflog --no-decorate $branch)"
 end_test
+
+##### fast-foward test support
+
+make_test_repo_with_two_branches() {
+    make_test_repo
+
+    #   Test branch to which we commit
+    $git branch $branch
+    assert_branch "737b0f439051 refs/heads/$branch"
+    touch $files/one; $git commit-filetree $branch $files
+    assert_branch "8a4cf12bef5f refs/heads/$branch"
+
+    #   Test tracking branch with an additional commit
+    $git branch $branch-tracking $branch
+    assert_branch "8a4cf12bef5f refs/heads/$branch-tracking" $branch-tracking
+    touch $files/two; $git commit-filetree $branch-tracking $files
+    assert_branch "fcb13b95f172 refs/heads/$branch-tracking" $branch-tracking
+}
+
+#   If you want to view the commit graph in a test, add the following.
+view_commit_graph() {
+    $git log --all --graph --pretty=oneline --abbrev=12
+}
+
+##### 10
+
+start_test 'Fast-forward commit branch'
+make_test_repo_with_two_branches
+
+#   Make test branch track test-tracking branch, but it's one commit behind.
+$git >/dev/null branch --set-upstream-to=$branch-tracking $branch
+assert_branch "8a4cf12bef5f refs/heads/$branch"
+
+touch $files/three
+test_equal 0 "$($git commit-filetree 2>&1 $branch $files; echo $?)"
+#   Parent commit is head of tracking branch.
+expected_log="
+ef65bb4d9108 978307200
+fcb13b95f172 978307200
+8a4cf12bef5f 978307200"
+test_equal "$expected_log" \
+    "$(echo; $git log -3 --abbrev=12 --pretty='format:%h %ct' $branch)"
+
+#   We can add more commits to commit branch when already ahead of tracking
+touch $files/four
+test_equal 0 "$($git commit-filetree 2>&1 $branch $files; echo $?)"
+expected_log="
+191c80c3688d 978307200
+ef65bb4d9108 978307200
+fcb13b95f172 978307200
+8a4cf12bef5f 978307200"
+test_equal "$expected_log" \
+    "$(echo; $git log -4 --abbrev=12 --pretty='format:%h %ct' $branch)"
+
+end_test
+
+##### 11
+
+start_test 'Cannot fast-forward commit branch'
+make_test_repo_with_two_branches
+
+#   Add another commit to local branch that's not on tracking branch.
+touch $files/commit-from-elsewhere
+$git commit-filetree $branch $files
+assert_branch "58cce3125df7 refs/heads/$branch"
+
+#   Make test branch track test-tracking branch, but 1/1 ahead/behind
+$git >/dev/null branch --set-upstream-to=$branch-tracking $branch
+
+touch $files/three
+exitcode="$($git commit-filetree $branch $files >/dev/null 2>&1; echo $?)"
+test_equal 3 "$exitcode"
+message=$($git commit-filetree 2>&1 $branch $files || true)
+expected='Branch testbr has diverged from tracking refs/heads/testbr-tracking'
+[[ $message =~ $expected ]] || fail_test "bad message: $message"
+
+end_test