diff --git a/CHANGELOG.md b/CHANGELOG.md
index d24d347f..3a657701 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,9 @@
 
 ## Unreleased
 
+### Added
+- Allow `bashunit::spy` to accept an optional exit code (e.g. `bashunit::spy thing 1`) or custom implementation function (e.g. `bashunit::spy thing mock_thing`) (#600)
+
 ### Fixed
 - Fix spying on `echo` or `printf` causing bashunit to hang due to infinite recursion (#607)
 
diff --git a/src/test_doubles.sh b/src/test_doubles.sh
index df4f60a3..c1d6d253 100644
--- a/src/test_doubles.sh
+++ b/src/test_doubles.sh
@@ -44,6 +44,7 @@ function bashunit::mock() {
 
 function bashunit::spy() {
   local command=$1
+  local exit_code_or_impl="${2:-}"
   local variable
   variable="$(bashunit::helper::normalize_variable_name "$command")"
 
@@ -56,6 +57,13 @@ function bashunit::spy() {
   export "${variable}_times_file"="$times_file"
   export "${variable}_params_file"="$params_file"
 
+  local body_suffix=""
+  if [[ "$exit_code_or_impl" =~ ^[0-9]+$ ]]; then
+    body_suffix="return $exit_code_or_impl"
+  elif [ -n "$exit_code_or_impl" ]; then
+    body_suffix="$exit_code_or_impl \"\$@\""
+  fi
+
   eval "function $command() {
     local raw=\"\$*\"
     local serialized=\"\"
@@ -69,6 +77,7 @@ function bashunit::spy() {
     _c=\$(cat '$times_file' 2>/dev/null || builtin echo 0)
     _c=\$((_c+1))
     builtin echo \"\$_c\" > '$times_file'
+    $body_suffix
   }"
 
   export -f "${command?}"
diff --git a/tests/unit/test_doubles_test.sh b/tests/unit/test_doubles_test.sh
index 9a6e6530..85a90d5f 100644
--- a/tests/unit/test_doubles_test.sh
+++ b/tests/unit/test_doubles_test.sh
@@ -216,3 +216,54 @@ function test_unsuccessful_spy_nth_called_with_invalid_index() {
 }
 
 
+
+function test_spy_with_exit_code_returns_specified_exit_code() {
+  bashunit::spy ps 1
+
+  local actual_exit_code=0
+  ps || actual_exit_code=$?
+
+  assert_have_been_called ps
+  assert_same "1" "$actual_exit_code"
+}
+
+function test_spy_with_exit_code_zero_returns_zero() {
+  bashunit::spy ps 0
+
+  ps
+  local actual_exit_code=$?
+
+  assert_have_been_called ps
+  assert_same "0" "$actual_exit_code"
+}
+
+function test_spy_with_impl_calls_custom_function() {
+  custom_ps_impl() {
+    builtin echo "custom output"
+  }
+  export -f custom_ps_impl
+
+  bashunit::spy ps custom_ps_impl
+
+  local output
+  output=$(ps)
+
+  assert_have_been_called ps
+  assert_same "custom output" "$output"
+}
+
+function test_spy_with_impl_records_calls_and_delegates() {
+  custom_ps_impl() {
+    builtin echo "delegated"
+  }
+  export -f custom_ps_impl
+
+  bashunit::spy ps custom_ps_impl
+
+  ps first
+  ps second
+
+  assert_have_been_called_times 2 ps
+  assert_have_been_called_nth_with 1 ps "first"
+  assert_have_been_called_nth_with 2 ps "second"
+}