diff --git a/core/modules/simpletest/composer.json b/core/modules/simpletest/composer.json
new file mode 100644
index 0000000..f0b3cf5
--- /dev/null
+++ b/core/modules/simpletest/composer.json
@@ -0,0 +1,10 @@
+{
+  "name": "drupal/simpletest",
+  "description": "Drupal testing framework.",
+  "keywords": ["drupal"],
+  "homepage": "https://www.drupal.org/project/drupal",
+  "license": "GPL-2.0+",
+  "require-dev": {
+    "phpunit/phpunit": "~4.8"
+  }
+}
diff --git a/core/modules/simpletest/simpletest.install b/core/modules/simpletest/simpletest.install
index c1422e6..0e3cd00 100644
--- a/core/modules/simpletest/simpletest.install
+++ b/core/modules/simpletest/simpletest.install
@@ -18,9 +18,19 @@
 function simpletest_requirements($phase) {
   $requirements = array();
 
+  $has_phpunit = class_exists('\PHPUnit_Framework_TestCase');
   $has_curl = function_exists('curl_init');
   $open_basedir = ini_get('open_basedir');
 
+  $requirements['phpunit'] = array(
+    'title' => t('PHPUnit dependency'),
+    'value' => $has_phpunit ? t('Found') : t('Not found'),
+  );
+  if (!$has_phpunit) {
+    $requirements['phpunit']['severity'] = REQUIREMENT_ERROR;
+    $requirements['phpunit']['description'] = t("The testing framework requires the PHPUnit package. Please run 'composer install --dev' to ensure it is present.");
+  }
+
   $requirements['curl'] = array(
     'title' => t('cURL'),
     'value' => $has_curl ? t('Enabled') : t('Not found'),
diff --git a/core/scripts/run-tests.sh b/core/scripts/run-tests.sh
index 15e6e80..f08190f 100755
--- a/core/scripts/run-tests.sh
+++ b/core/scripts/run-tests.sh
@@ -34,6 +34,11 @@
 const SIMPLETEST_SCRIPT_EXIT_FAILURE = 1;
 const SIMPLETEST_SCRIPT_EXIT_EXCEPTION = 2;
 
+if (!class_exists('\PHPUnit_Framework_TestCase')) {
+  echo "\nrun-tests.sh requires the PHPUnit testing framework. Please use 'composer install --dev' to ensure that it is present.\n\n";
+  exit(SIMPLETEST_SCRIPT_EXIT_FAILURE);
+}
+
 // Set defaults and get overrides.
 list($args, $count) = simpletest_script_parse_args();