diff --git a/lib/diff b/lib/diff
new file mode 100644
index 0000000..c6f3271
--- /dev/null
+++ b/lib/diff
@@ -0,0 +1,120 @@
+#! /usr/bin/env bash
+#
+# Tools for examining file differences
+#
+# Exports:
+#   @go.diff_check_editor
+#     Checks that `_GO_DIFF_EDITOR` is set
+#
+#   @go.diff_files
+#     Log differences between two files
+#
+#   @go.diff_directories
+#     Log differences between two directory structures
+
+export _GO_DIFF_EDITOR="${_GO_DIFF_EDITOR:-vimdiff}"
+
+. "$_GO_USE_MODULES" 'log' 'path'
+
+# Checks that `_GO_DIFF_EDITOR` is set
+#
+# Used by the functions in this module when the `--edit` flag is specified.
+# Fails with a stack trace if it isn't set.
+@go.diff_check_editor() {
+  if [[ -z "$_GO_DIFF_EDITOR" ]]; then
+    @go.log FATAL "_GO_DIFF_EDITOR not defined"
+  elif ! command -v "$_GO_DIFF_EDITOR" >/dev/null; then
+    @go.log FATAL "_GO_DIFF_EDITOR not installed: $_GO_DIFF_EDITOR"
+  fi
+}
+
+# Log differences between two files
+#
+# Options:
+#   --edit:  Open `_GO_DIFF_EDITOR` on the files if they differ
+#
+# Arguments:
+#   lhs:  The "left-hand side" file
+#   rhs:  The "right-hand side" file
+@go.diff_files() {
+  local lhs
+  local rhs
+  local edit
+
+  if [[ "$1" == '--edit' ]]; then
+    @go.diff_check_editor
+    edit='true'
+    shift
+  fi
+
+  lhs="$1"
+  rhs="$2"
+
+  if [[ ! -f "$lhs" ]]; then
+    @go.log WARN "Left-hand side file $1 doesn't exist or isn't a regular file"
+  elif [[ ! -f "$rhs" ]]; then
+    @go.log WARN "Right-hand side file $2 doesn't exist or isn't a regular file"
+  elif ! diff "$lhs" "$rhs" >/dev/null; then
+    @go.log WARN "$lhs" differs from "$rhs"
+    if [[ -n "$edit" ]]; then
+      @go.log INFO "Editing $lhs and $rhs"
+      "$_GO_DIFF_EDITOR" "$lhs" "$rhs"
+    fi
+  else
+    return '0'
+  fi
+  return '1'
+}
+
+# Log differences between two directory structures
+#
+# Note that files and directories from `lhs_dir` that are missing in `rhs_dir`
+# are reported, but not the other way around.
+#
+# Options:
+#   --edit:  Open `_GO_DIFF_EDITOR` on any files that differ between the dirs
+#
+# Arguments:
+#   lhs_dir:  The "left-hand side" directory
+#   rhs_dir:  The "right-hand side" directory
+@go.diff_directories() {
+  local __go_lhs_dir
+  local __go_rhs_dir
+  local __go_diff_files_args=()
+  local __go_diff_directories_result='0'
+
+  if [[ "$1" == '--edit' ]]; then
+    __go_diff_files_args=('--edit')
+    shift
+  fi
+
+  __go_lhs_dir="$1"
+  __go_rhs_dir="$2"
+
+  if [[ ! -d "$__go_lhs_dir" ]]; then
+    @go.log WARN \
+      "Left-hand side directory $1 doesn't exist or isn't a directory"
+  elif [[ ! -d "$__go_rhs_dir" ]]; then
+    @go.log WARN \
+      "Right-hand side directory $2 doesn't exist or isn't a directory"
+  else
+    @go.walk_file_system _@go.diff_directories_impl "$__go_lhs_dir" || :
+    return "$__go_diff_directories_result"
+  fi
+  return '1'
+}
+
+# --------------------------------
+# IMPLEMENTATION - HERE BE DRAGONS
+#
+# None of the functions below this line are part of the public interface.
+# --------------------------------
+_@go.diff_directories_impl() {
+  local lhs="$1"
+  local rhs="${__go_rhs_dir}${lhs#$__go_lhs_dir}"
+
+  if [[ -f "$lhs" ]] &&
+    ! @go.diff_files "${__go_diff_files_args[@]}" "$lhs" "$rhs"; then
+    __go_diff_directories_result='1'
+  fi
+}
diff --git a/tests/diff/check-diff-editor.bats b/tests/diff/check-diff-editor.bats
new file mode 100644
index 0000000..adc40cc
--- /dev/null
+++ b/tests/diff/check-diff-editor.bats
@@ -0,0 +1,37 @@
+#! /usr/bin/env bats
+
+load ../environment
+
+setup() {
+  test_filter
+  @go.create_test_go_script \
+    '. "$_GO_USE_MODULES" "diff"'\
+    '@go.diff_check_editor'
+}
+
+teardown() {
+  @go.remove_test_go_rootdir
+}
+
+@test "$SUITE: fails if __GO_DIFF_EDITOR not defined" {
+  @go.create_test_go_script \
+    '. "$_GO_USE_MODULES" "diff"'\
+    '_GO_DIFF_EDITOR= @go.diff_check_editor'
+  run "$TEST_GO_SCRIPT"
+  assert_failure
+  assert_output_matches 'FATAL.* _GO_DIFF_EDITOR not defined'
+}
+
+@test "$SUITE: fails if _GO_DIFF_EDITOR not found" {
+  _GO_DIFF_EDITOR='nonexistent-editor' run "$TEST_GO_SCRIPT"
+  assert_failure
+  assert_output_matches \
+    'FATAL.* _GO_DIFF_EDITOR not installed: nonexistent-editor'
+}
+
+@test "$SUITE: does nothing if _GO_DIFF_EDITOR found" {
+  stub_program_in_path 'vimdiff'
+  _GO_DIFF_EDITOR='vimdiff' run "$TEST_GO_SCRIPT"
+  restore_program_in_path 'vimdiff'
+  assert_success ''
+}
diff --git a/tests/diff/diff-directories.bats b/tests/diff/diff-directories.bats
new file mode 100644
index 0000000..1df1421
--- /dev/null
+++ b/tests/diff/diff-directories.bats
@@ -0,0 +1,107 @@
+#! /usr/bin/env bats
+
+load ../environment
+
+LHS_DIR=
+RHS_DIR=
+
+setup() {
+  test_filter
+  @go.create_test_go_script \
+    '. "$_GO_USE_MODULES" "diff"' \
+    '@go.diff_directories "$@"'
+
+  LHS_DIR="$TEST_GO_ROOTDIR/lhs"
+  RHS_DIR="$TEST_GO_ROOTDIR/rhs"
+  mkdir -p "$LHS_DIR" "$RHS_DIR"
+}
+
+teardown() {
+  @go.remove_test_go_rootdir
+}
+
+@test "$SUITE: warns if LHS dir doesn't exist" {
+  local lhs="$LHS_DIR"
+  rmdir "$lhs"
+  run "$TEST_GO_SCRIPT" "$lhs" "$RHS_DIR"
+  assert_failure
+  assert_output_matches \
+    "WARN.* Left-hand side directory $lhs doesn't exist or isn't a directory"
+}
+
+@test "$SUITE: warns if RHS dir doesn't exist" {
+  local rhs="$RHS_DIR"
+  rmdir "$rhs"
+  run "$TEST_GO_SCRIPT" "$LHS_DIR" "$rhs"
+  assert_failure
+  assert_output_matches \
+    "WARN.* Right-hand side directory $rhs doesn't exist or isn't a directory"
+}
+
+@test "$SUITE: does nothing if first directory empty" {
+  printf '%s\n' 'foo' >"$RHS_DIR/foo"
+  run "$TEST_GO_SCRIPT" "$LHS_DIR" "$RHS_DIR"
+  assert_success ''
+}
+
+@test "$SUITE: returns success if directories contain the same single file" {
+  skip_if_system_missing 'diff'
+  printf '%s\n' 'foo' >"$LHS_DIR/foo"
+  printf '%s\n' 'foo' >"$RHS_DIR/foo"
+  run "$TEST_GO_SCRIPT" "$LHS_DIR" "$RHS_DIR"
+  assert_success ''
+}
+
+@test "$SUITE: returns success if rhs contains more files" {
+  skip_if_system_missing 'diff'
+  printf '%s\n' 'foo' >"$LHS_DIR/foo"
+  printf '%s\n' 'foo' >"$RHS_DIR/foo"
+  printf '%s\n' 'bar' >"$RHS_DIR/bar"
+  run "$TEST_GO_SCRIPT" "$LHS_DIR" "$RHS_DIR"
+  assert_success ''
+}
+
+@test "$SUITE: returns an error if rhs contains fewer files" {
+  skip_if_system_missing 'diff'
+  local missing="$RHS_DIR/bar"
+
+  printf '%s\n' 'foo' >"$LHS_DIR/foo"
+  printf '%s\n' 'bar' >"$LHS_DIR/bar"
+  printf '%s\n' 'foo' >"$RHS_DIR/foo"
+
+  run "$TEST_GO_SCRIPT" "$LHS_DIR" "$RHS_DIR"
+  assert_failure
+  assert_output_matches \
+    "WARN.* Right-hand side file $missing doesn't exist or isn't a regular file"
+}
+
+@test "$SUITE: warns and returns an error when files differ" {
+  skip_if_system_missing 'diff'
+
+  printf '%s\n' 'foo' >"$LHS_DIR/foo"
+  printf '%s\n' 'bar' >"$RHS_DIR/foo"
+
+  run "$TEST_GO_SCRIPT" "$LHS_DIR" "$RHS_DIR"
+  assert_failure
+  assert_output_matches "WARN.* $LHS_DIR/foo differs from $RHS_DIR/foo"
+}
+
+@test "$SUITE: --edit opens _GO_DIFF_EDITOR when files differ" {
+  skip_if_system_missing 'diff'
+
+  printf '%s\n' 'foo' >"$LHS_DIR/foo"
+  printf '%s\n' 'bar' >"$RHS_DIR/foo"
+
+  stub_program_in_path 'vimdiff' \
+    'printf "%s\n" "LHS: $1" "RHS: $2"'
+
+  _GO_DIFF_EDITOR='vimdiff' run "$TEST_GO_SCRIPT" --edit "$LHS_DIR" "$RHS_DIR"
+  restore_program_in_path 'vimdiff'
+
+  assert_failure
+  assert_lines_match \
+    "WARN.* $LHS_DIR/foo differs from $RHS_DIR/foo" \
+    "INFO.* Editing $LHS_DIR/foo and $RHS_DIR/foo" \
+    "LHS: $LHS_DIR/foo" \
+    "RHS: $RHS_DIR/foo"
+}
diff --git a/tests/diff/diff-files.bats b/tests/diff/diff-files.bats
new file mode 100644
index 0000000..77976bc
--- /dev/null
+++ b/tests/diff/diff-files.bats
@@ -0,0 +1,90 @@
+#! /usr/bin/env bats
+
+load ../environment
+
+setup() {
+  test_filter
+  @go.create_test_go_script \
+    '. "$_GO_USE_MODULES" "diff"'\
+    '@go.diff_files "$@"'
+}
+
+teardown() {
+  @go.remove_test_go_rootdir
+}
+
+@test "$SUITE: warns if lhs file doesn't exist or isn't regular" {
+  local lhs="$TEST_GO_ROOTDIR/foo"
+
+  run "$TEST_GO_SCRIPT" "$lhs"
+  assert_failure
+  assert_output_matches \
+    "WARN.* Left-hand side file $lhs doesn't exist or isn't a regular file"
+}
+
+@test "$SUITE: warns if rhs file doesn't exist or isn't regular" {
+  local lhs="$TEST_GO_ROOTDIR/foo"
+  local rhs="$TEST_GO_ROOTDIR/bar"
+
+  mkdir -p "$TEST_GO_ROOTDIR"
+  printf '%s\n' 'foo' >"$lhs"
+  mkdir "$rhs"
+
+  run "$TEST_GO_SCRIPT" "$lhs" "$rhs"
+  assert_failure
+  assert_output_matches \
+    "WARN.* Right-hand side file $rhs doesn't exist or isn't a regular file"
+}
+
+@test "$SUITE: return success if the files match" {
+  skip_if_system_missing 'diff'
+
+  local lhs="$TEST_GO_ROOTDIR/foo"
+  local rhs="$TEST_GO_ROOTDIR/bar"
+
+  mkdir -p "$TEST_GO_ROOTDIR"
+  printf '%s\n' 'foo' >"$lhs"
+  printf '%s\n' 'foo' >"$rhs"
+
+  run "$TEST_GO_SCRIPT" "$lhs" "$rhs"
+  assert_success ''
+}
+
+@test "$SUITE: warns and return failure if the files differ" {
+  skip_if_system_missing 'diff'
+
+  local lhs="$TEST_GO_ROOTDIR/foo"
+  local rhs="$TEST_GO_ROOTDIR/bar"
+
+  mkdir -p "$TEST_GO_ROOTDIR"
+  printf '%s\n' 'foo' >"$lhs"
+  printf '%s\n' 'bar' >"$rhs"
+
+  run "$TEST_GO_SCRIPT" "$lhs" "$rhs"
+  assert_failure
+  assert_output_matches "WARN.* $lhs differs from $rhs"
+}
+
+@test "$SUITE: --edit opens _GO_DIFF_EDITOR if the files differ" {
+  skip_if_system_missing 'diff'
+
+  local lhs="$TEST_GO_ROOTDIR/foo"
+  local rhs="$TEST_GO_ROOTDIR/bar"
+
+  mkdir -p "$TEST_GO_ROOTDIR"
+  printf '%s\n' 'foo' >"$lhs"
+  printf '%s\n' 'bar' >"$rhs"
+
+  stub_program_in_path 'vimdiff' \
+    'printf "%s\n" "LHS: $1" "RHS: $2"'
+
+  _GO_DIFF_EDITOR='vimdiff' run "$TEST_GO_SCRIPT" --edit "$lhs" "$rhs"
+  restore_program_in_path 'vimdiff'
+
+  assert_failure
+  assert_lines_match \
+    "WARN.* $lhs differs from $rhs" \
+    "INFO.* Editing $lhs and $rhs" \
+    "LHS: $lhs" \
+    "RHS: $rhs"
+}