diff --git a/core/phpunit.xml.dist b/core/phpunit.xml.dist
index 951bd55..84f1810 100644
--- a/core/phpunit.xml.dist
+++ b/core/phpunit.xml.dist
@@ -9,6 +9,9 @@
  - -printer="\Drupal\Tests\Listeners\HtmlOutputPrinter" to use it (note there
  should be no spaces between the hyphens).
 -->
+<!-- PHPUnit expects functional tests to be run with either a priviledged user
+ or your Unix user. See core/tests/README.md for details.
+-->
 <phpunit bootstrap="tests/bootstrap.php" colors="true"
          beStrictAboutTestsThatDoNotTestAnything="true"
          beStrictAboutOutputDuringTests="true"
diff --git a/core/tests/README.md b/core/tests/README.md
index dfd1154..7d64223 100644
--- a/core/tests/README.md
+++ b/core/tests/README.md
@@ -13,3 +13,17 @@
   ./vendor/bin/phpunit -c core --testsuite functional
   ./vendor/bin/phpunit -c core --testsuite functional-javascript
   ```
+
+Note: to run successfully, functional tests need to know under which user your
+webserver is running. You can either configure Apache to run as your own user
+or run tests as a privileged user instead.
+
+E.g. if you're running Apache and the default user is `www-data`, the above
+functional tests will thus have to be invoked this way:
+
+```
+export SIMPLETEST_DB='mysql://root@localhost/dev_d8'
+export SIMPLETEST_BASE_URL='http://d8.dev'
+sudo -u www-data ./vendor/bin/phpunit -c core --testsuite functional
+sudo -u www-data ./vendor/bin/phpunit -c core --testsuite functional-javascript
+```