diff --git a/core/lib/Drupal/Core/Form/FormStateInterface.php b/core/lib/Drupal/Core/Form/FormStateInterface.php
index ebd2ec6..7706dc2 100644
--- a/core/lib/Drupal/Core/Form/FormStateInterface.php
+++ b/core/lib/Drupal/Core/Form/FormStateInterface.php
@@ -377,7 +377,23 @@ public function hasValue($key);
   public function isValueEmpty($key);
 
   /**
-   * @todo.
+   * Gets the form value that corresponds to a given form element.
+   *
+   * @param array $element
+   *   The form element that should have its value retrieved; in most cases you
+   *   can just pass in the element from the $form array, although the only
+   *   component that is actually used is '#parents'. If constructing yourself,
+   *   set $element['#parents'] to be an array giving the path through the form
+   *   array's keys to the element whose value you want to retrieve. For
+   *   instance, if you want to retrieve the value of $form['elem1']['elem2'],
+   *   which should be stored in $form_state->getValue(['elem1', 'elem2']), you
+   *   would set $element['#parents'] = ['elem1', 'elem2'].
+   * @param bool $exists
+   *   (optional) If given, an already defined variable that is altered by
+   *   reference.
+   *
+   * @return mixed
+   *   The value for the form element, if it exists, NULL otherwise.
    */
   public function getValueForElement(array $element, &$exists = NULL);
 
diff --git a/core/tests/Drupal/Tests/Core/Form/FormStateTest.php b/core/tests/Drupal/Tests/Core/Form/FormStateTest.php
index 24904e3..3e6cf48 100644
--- a/core/tests/Drupal/Tests/Core/Form/FormStateTest.php
+++ b/core/tests/Drupal/Tests/Core/Form/FormStateTest.php
@@ -143,6 +143,47 @@ public function testFormErrorsDuringSubmission() {
   }
 
   /**
+   * @covers ::getValueForElement
+   *
+   * @dataProvider providerTestGetValueForElement
+   */
+  public function testGetValueForElement($set_data, $expected) {
+    $element = [
+      '#parents' => [
+        'foo',
+        'bar',
+      ],
+    ];
+    $value = $this->randomMachineName();
+
+    $input_exists = NULL;
+    $form_state = new FormState();
+    if ($set_data) {
+      $form_state->setValue(['foo', 'bar'], $value);
+    }
+
+    $value_from_form_state = $form_state->getValueForElement($element, $input_exists);
+    if ($set_data) {
+      $this->assertSame($value, $value_from_form_state);
+    }
+    else {
+      $this->assertNull($value_from_form_state);
+    }
+    $this->assertSame($expected, $input_exists);
+  }
+
+  public function providerTestGetValueForElement() {
+    $data = [];
+    $data[] = [
+      FALSE, FALSE,
+    ];
+    $data[] = [
+      TRUE, TRUE,
+    ];
+    return $data;
+  }
+
+  /**
    * Tests that setting the value for an element adds to the values.
    *
    * @covers ::setValueForElement