diff --git a/src/assert_equal.bash b/src/assert_equal.bash
index 4ef1297..c6e5cc0 100644
--- a/src/assert_equal.bash
+++ b/src/assert_equal.bash
@@ -3,11 +3,12 @@
 #
 # Summary: Fail if the actual and expected values are not equal.
 #
-# Usage: assert_equal <actual> <expected>
+# Usage: assert_equal [-d] <actual> <expected>
 #
 # Options:
 #   <actual>      The value being compared.
 #   <expected>    The value to compare against.
+#   -d, --diff     Show diff between `expected` and `$output`
 #
 #   ```bash
 #   @test 'assert_equal()' {
@@ -31,12 +32,29 @@
 #   actual   : have
 #   --
 #   ```
+#
+# If the `--diff` option is set, a diff between the expected and actual output is shown.
 assert_equal() {
+  local -i show_diff=0
+
+  while (( $# > 0 )); do
+    case "$1" in
+    -d|--diff) show_diff=$(( $# > 1 )); shift ;;
+    *) break ;;
+    esac
+  done
+
   if [[ $1 != "$2" ]]; then
-    batslib_print_kv_single_or_multi 8 \
-    'expected' "$2" \
-    'actual'   "$1" \
-    | batslib_decorate 'values do not equal' \
-    | fail
+    if (( show_diff )); then
+      diff -u <(echo "$1") <(echo "$2") | tail -n +3 \
+      | batslib_decorate 'values do not equal' \
+      | fail
+    else
+       batslib_print_kv_single_or_multi 8 \
+      'expected' "$2" \
+      'actual'   "$1" \
+      | batslib_decorate 'values do not equal' \
+      | fail
+    fi
   fi
 }
diff --git a/src/assert_output.bash b/src/assert_output.bash
index 82d0a0c..8e9739c 100644
--- a/src/assert_output.bash
+++ b/src/assert_output.bash
@@ -3,11 +3,12 @@
 #
 # Summary: Fail if `$output' does not match the expected output.
 #
-# Usage: assert_output [-p | -e] [- | [--] <expected>]
+# Usage: assert_output [-p | -e | -d] [- | [--] <expected>]
 #
 # Options:
 #   -p, --partial  Match if `expected` is a substring of `$output`
 #   -e, --regexp   Treat `expected` as an extended regular expression
+#   -d, --diff     Show diff between `expected` and `$output`
 #   -, --stdin     Read `expected` value from STDIN
 #   <expected>     The expected value, substring or regular expression
 #
@@ -57,6 +58,8 @@
 #   --
 #   ```
 #
+# If the `--diff` option is set, a diff between the expected and actual output is shown.
+#
 # ## Existence
 #
 # To assert that any output exists at all, omit the `expected` argument.
@@ -126,6 +129,7 @@ assert_output() {
   local -i is_mode_regexp=0
   local -i is_mode_nonempty=0
   local -i use_stdin=0
+  local -i show_diff=0
   : "${output?}"
 
   # Handle options.
@@ -137,6 +141,7 @@ assert_output() {
     case "$1" in
     -p|--partial) is_mode_partial=1; shift ;;
     -e|--regexp) is_mode_regexp=1; shift ;;
+    -d|--diff) show_diff=1; shift ;;
     -|--stdin) use_stdin=1; shift ;;
     --) shift; break ;;
     *) break ;;
@@ -185,6 +190,16 @@ assert_output() {
       | batslib_decorate 'output does not contain substring' \
       | fail
     fi
+  elif (( show_diff )); then
+    if (( $# == 0 )); then
+      echo "Missing expected output" \
+      | batslib_decorate 'ERROR: assert_output' \
+      | fail
+    elif [[ $output != "$expected" ]]; then
+      diff -u <(echo "$output") <(echo "$expected") | tail -n +3 \
+      | batslib_decorate 'output differs' \
+      | fail
+    fi
   else
     if [[ $output != "$expected" ]]; then
       batslib_print_kv_single_or_multi 8 \
diff --git a/test/assert_equal.bats b/test/assert_equal.bats
index 7d3fc86..f4ab3b5 100755
--- a/test/assert_equal.bats
+++ b/test/assert_equal.bats
@@ -60,3 +60,21 @@ actual   : a
 --
 ERR_MSG
 }
+
+@test 'assert_equal() -d <actual> <expected>: returns 0 if <actual> equals <expected>' {
+  run assert_equal -d 'a' 'a'
+  assert_test_pass
+}
+
+@test 'assert_equal() -d <actual> <expected>: returns 1 and displays details if <actual> does not equal <expected>' {
+  run assert_equal -d 'a' 'b'
+
+  assert_test_fail <<'ERR_MSG'
+
+-- values do not equal --
+@@ -1 +1 @@
+-a
++b
+--
+ERR_MSG
+}
diff --git a/test/assert_output.bats b/test/assert_output.bats
index e9609e0..8c79342 100755
--- a/test/assert_output.bats
+++ b/test/assert_output.bats
@@ -13,6 +13,12 @@ load test_helper
   assert_test_pass
 }
 
+@test "assert_output() -d <expected>: returns 0 if <expected> equals \`\$output'" {
+  run echo 'a'
+  run assert_output -d 'a'
+  assert_test_pass
+}
+
 @test "assert_output() <expected>: returns 1 and displays details if <expected> does not equal \`\$output'" {
   run echo 'b'
   run assert_output 'a'
@@ -26,6 +32,20 @@ actual   : b
 ERR_MSG
 }
 
+@test "assert_output() -d <expected>: returns 1 and displays details if <expected> does not equal \`\$output'" {
+  run echo 'b'
+  run assert_output -d 'a'
+
+  assert_test_fail <<'ERR_MSG'
+
+-- output differs --
+@@ -1 +1 @@
+-b
++a
+--
+ERR_MSG
+}
+
 @test 'assert_output(): succeeds if output is non-empty' {
   run echo 'a'
   run assert_output
@@ -262,7 +282,6 @@ Invalid extended regular expression: `[.*'
 ERR_MSG
 }
 
-
 #
 # Common
 #