diff --git src/wp-includes/cron.php src/wp-includes/cron.php
index fbf5a629af..e66ea346e1 100644
--- src/wp-includes/cron.php
+++ src/wp-includes/cron.php
@@ -21,12 +21,14 @@
  * Use wp_schedule_event() to schedule a recurring event.
  *
  * @since 2.1.0
+ * @since 5.0.0 Return value modified to boolean indicating success or failure.
+ *
  * @link https://codex.wordpress.org/Function_Reference/wp_schedule_single_event
  *
  * @param int    $timestamp  Unix timestamp (UTC) for when to next run the event.
  * @param string $hook       Action hook to execute when the event is run.
  * @param array  $args       Optional. Array containing each separate argument to pass to the hook's callback function.
- * @return false|void False if the event did not get scheduled.
+ * @return bool True if event successfully scheduled. False for failure.
  */
 function wp_schedule_single_event( $timestamp, $hook, $args = array() ) {
 	// Make sure timestamp is a positive integer
@@ -77,7 +79,7 @@ function wp_schedule_single_event( $timestamp, $hook, $args = array() ) {
 		'args'     => $event->args,
 	);
 	uksort( $crons, 'strnatcasecmp' );
-	_set_cron_array( $crons );
+	return _set_cron_array( $crons );
 }
 
 /**
@@ -99,13 +101,15 @@ function wp_schedule_single_event( $timestamp, $hook, $args = array() ) {
  * Use wp_schedule_single_event() to schedule a non-recurring event.
  *
  * @since 2.1.0
+ * @since 5.0.0 Return value modified to boolean indicating success or failure.
+ *
  * @link https://codex.wordpress.org/Function_Reference/wp_schedule_event
  *
  * @param int    $timestamp  Unix timestamp (UTC) for when to next run the event.
  * @param string $recurrence How often the event should subsequently recur. See wp_get_schedules() for accepted values.
  * @param string $hook       Action hook to execute when the event is run.
  * @param array  $args       Optional. Array containing each separate argument to pass to the hook's callback function.
- * @return false|void False if the event did not get scheduled.
+ * @return bool True if event successfully scheduled. False for failure.
  */
 function wp_schedule_event( $timestamp, $recurrence, $hook, $args = array() ) {
 	// Make sure timestamp is a positive integer
@@ -143,19 +147,20 @@ function wp_schedule_event( $timestamp, $recurrence, $hook, $args = array() ) {
 		'interval' => $event->interval,
 	);
 	uksort( $crons, 'strnatcasecmp' );
-	_set_cron_array( $crons );
+	return _set_cron_array( $crons );
 }
 
 /**
  * Reschedules a recurring event.
  *
  * @since 2.1.0
+ * @since 5.0.0 Return value modified to boolean indicating success or failure.
  *
  * @param int    $timestamp  Unix timestamp (UTC) for when to next run the event.
  * @param string $recurrence How often the event should subsequently recur. See wp_get_schedules() for accepted values.
  * @param string $hook       Action hook to execute when the event is run.
  * @param array  $args       Optional. Array containing each separate argument to pass to the hook's callback function.
- * @return false|void False if the event did not get rescheduled.
+ * @return bool True if event successfully rescheduled. False for failure.
  */
 function wp_reschedule_event( $timestamp, $recurrence, $hook, $args = array() ) {
 	// Make sure timestamp is a positive integer
@@ -189,7 +194,7 @@ function wp_reschedule_event( $timestamp, $recurrence, $hook, $args = array() )
 		$timestamp = $now + ( $interval - ( ( $now - $timestamp ) % $interval ) );
 	}
 
-	wp_schedule_event( $timestamp, $recurrence, $hook, $args );
+	return wp_schedule_event( $timestamp, $recurrence, $hook, $args );
 }
 
 /**
@@ -199,13 +204,14 @@ function wp_reschedule_event( $timestamp, $recurrence, $hook, $args = array() )
  * identified.
  *
  * @since 2.1.0
+ * @since 5.0.0 Return value modified to boolean indicating success or failure.
  *
  * @param int    $timestamp Unix timestamp (UTC) of the event.
  * @param string $hook      Action hook of the event.
  * @param array  $args      Optional. Array containing each separate argument to pass to the hook's callback function.
  *                          Although not passed to a callback, these arguments are used to uniquely identify the
  *                          event, so they should be the same as those used when originally scheduling the event.
- * @return false|void False if the event did not get unscheduled.
+ * @return bool True if event successfully unscheduled. False for failure.
  */
 function wp_unschedule_event( $timestamp, $hook, $args = array() ) {
 	// Make sure timestamp is a positive integer
@@ -222,16 +228,25 @@ function wp_unschedule_event( $timestamp, $hook, $args = array() ) {
 	if ( empty( $crons[ $timestamp ] ) ) {
 		unset( $crons[ $timestamp ] );
 	}
-	_set_cron_array( $crons );
+	return _set_cron_array( $crons );
 }
 
 /**
  * Unschedules all events attached to the hook with the specified arguments.
  *
+ * Warning: This function may return Boolean FALSE, but may also return a non-Boolean
+ * value which evaluates to FALSE. For information about casting to booleans see the
+ * {@link http://php.net/manual/en/language.types.boolean.php PHP documentation}. Use
+ * the `===` operator for testing the return value of this function.
+ *
  * @since 2.1.0
+ * @since 5.0.0 Return value modified to indicate success or failure.
  *
  * @param string $hook Action hook, the execution of which will be unscheduled.
  * @param array $args Optional. Arguments that were to be passed to the hook's callback function.
+ * @return bool|int On success an integer indicating number of events unscheduled (0 indicates no
+ *                  events were registered with the hook and arguments combination), false if
+ *                  unscheduling one or more events fail.
  */
 function wp_clear_scheduled_hook( $hook, $args = array() ) {
 	// Backward compatibility
@@ -246,15 +261,20 @@ function wp_clear_scheduled_hook( $hook, $args = array() ) {
 	// and, wp_next_scheduled() returns the same schedule in an infinite loop.
 	$crons = _get_cron_array();
 	if ( empty( $crons ) ) {
-		return;
+		return 0;
 	}
 
-	$key = md5( serialize( $args ) );
+	$results = array();
+	$key     = md5( serialize( $args ) );
 	foreach ( $crons as $timestamp => $cron ) {
 		if ( isset( $cron[ $hook ][ $key ] ) ) {
-			wp_unschedule_event( $timestamp, $hook, $args );
+			$results[] = wp_unschedule_event( $timestamp, $hook, $args );
 		}
 	}
+	if ( in_array( false, $results, true ) ) {
+		return false;
+	}
+	return count( $results );
 }
 
 /**
@@ -262,14 +282,29 @@ function wp_clear_scheduled_hook( $hook, $args = array() ) {
  *
  * Can be useful for plugins when deactivating to clean up the cron queue.
  *
+ * Warning: This function may return Boolean FALSE, but may also return a non-Boolean
+ * value which evaluates to FALSE. For information about casting to booleans see the
+ * {@link http://php.net/manual/en/language.types.boolean.php PHP documentation}. Use
+ * the `===` operator for testing the return value of this function.
+ *
  * @since 4.9.0
+ * @since 5.0.0 Return value added to indicate success or failure.
  *
  * @param string $hook Action hook, the execution of which will be unscheduled.
+ * @return bool|int On success an integer indicating number of events unscheduled (0 indicates no
+ *                  events were registered on the hook), false if unscheduling fails.
  */
 function wp_unschedule_hook( $hook ) {
 	$crons = _get_cron_array();
+	if ( empty( $crons ) ) {
+		return 0;
+	}
 
+	$results = array();
 	foreach ( $crons as $timestamp => $args ) {
+		if ( ! empty( $crons[ $timestamp ][ $hook ] ) ) {
+			$results[] = count( $crons[ $timestamp ][ $hook ] );
+		}
 		unset( $crons[ $timestamp ][ $hook ] );
 
 		if ( empty( $crons[ $timestamp ] ) ) {
@@ -277,7 +312,10 @@ function wp_unschedule_hook( $hook ) {
 		}
 	}
 
-	_set_cron_array( $crons );
+	if ( _set_cron_array( $crons ) ) {
+		return array_sum( $results );
+	}
+	return false;
 }
 
 /**
@@ -309,8 +347,11 @@ function wp_next_scheduled( $hook, $args = array() ) {
  * Sends a request to run cron through HTTP request that doesn't halt page loading.
  *
  * @since 2.1.0
+ * @since 5.0.0 Return values added.
  *
  * @param int $gmt_time Optional. Unix timestamp (UTC). Default 0 (current time is used).
+ * @return false|WP_Error|array False if no events need to run needed to run.
+ *                              When cron runs, return the result of {@see wp_remote_post}
  */
 function spawn_cron( $gmt_time = 0 ) {
 	if ( ! $gmt_time ) {
@@ -318,7 +359,7 @@ function spawn_cron( $gmt_time = 0 ) {
 	}
 
 	if ( defined( 'DOING_CRON' ) || isset( $_GET['doing_wp_cron'] ) ) {
-		return;
+		return false;
 	}
 
 	/*
@@ -336,23 +377,23 @@ function spawn_cron( $gmt_time = 0 ) {
 
 	// don't run if another process is currently running it or more than once every 60 sec.
 	if ( $lock + WP_CRON_LOCK_TIMEOUT > $gmt_time ) {
-		return;
+		return false;
 	}
 
 	//sanity check
 	$crons = _get_cron_array();
 	if ( ! is_array( $crons ) ) {
-		return;
+		return false;
 	}
 
 	$keys = array_keys( $crons );
 	if ( isset( $keys[0] ) && $keys[0] > $gmt_time ) {
-		return;
+		return false;
 	}
 
 	if ( defined( 'ALTERNATE_WP_CRON' ) && ALTERNATE_WP_CRON ) {
 		if ( 'GET' !== $_SERVER['REQUEST_METHOD'] || defined( 'DOING_AJAX' ) || defined( 'XMLRPC_REQUEST' ) ) {
-			return;
+			return false;
 		}
 
 		$doing_wp_cron = sprintf( '%.22F', $gmt_time );
@@ -368,7 +409,7 @@ function spawn_cron( $gmt_time = 0 ) {
 		flush();
 
 		WP_DEBUG ? include_once( ABSPATH . 'wp-cron.php' ) : @include_once( ABSPATH . 'wp-cron.php' );
-		return;
+		return false;
 	}
 
 	// Set the cron lock with the current unix timestamp, when the cron is being spawned.
@@ -409,31 +450,42 @@ function spawn_cron( $gmt_time = 0 ) {
 		), $doing_wp_cron
 	);
 
-	wp_remote_post( $cron_request['url'], $cron_request['args'] );
+	return wp_remote_post( $cron_request['url'], $cron_request['args'] );
 }
 
 /**
  * Run scheduled callbacks or spawn cron for all scheduled events.
  *
+ * Warning: This function may return Boolean FALSE, but may also return a non-Boolean
+ * value which evaluates to FALSE. For information about casting to booleans see the
+ * {@link http://php.net/manual/en/language.types.boolean.php PHP documentation}. Use
+ * the `===` operator for testing the return value of this function.
+ *
  * @since 2.1.0
+ * @since 5.0.0 Return value added to indicate success or failure.
+ *
+ * @return bool|int On success an integer indicating number of events spawned (0 indicates no
+ *                  events needed to be spawned), false if spawning fails for one or more events.
  */
 function wp_cron() {
 	// Prevent infinite loops caused by lack of wp-cron.php
 	if ( strpos( $_SERVER['REQUEST_URI'], '/wp-cron.php' ) !== false || ( defined( 'DISABLE_WP_CRON' ) && DISABLE_WP_CRON ) ) {
-		return;
+		return 0;
 	}
 
-	if ( false === $crons = _get_cron_array() ) {
-		return;
+	$crons = _get_cron_array();
+	if ( false === $crons ) {
+		return 0;
 	}
 
 	$gmt_time = microtime( true );
 	$keys     = array_keys( $crons );
 	if ( isset( $keys[0] ) && $keys[0] > $gmt_time ) {
-		return;
+		return 0;
 	}
 
 	$schedules = wp_get_schedules();
+	$results   = array();
 	foreach ( $crons as $timestamp => $cronhooks ) {
 		if ( $timestamp > $gmt_time ) {
 			break;
@@ -442,10 +494,19 @@ function wp_cron() {
 			if ( isset( $schedules[ $hook ]['callback'] ) && ! call_user_func( $schedules[ $hook ]['callback'] ) ) {
 				continue;
 			}
-			spawn_cron( $gmt_time );
+			if ( is_wp_error( spawn_cron( $gmt_time ) ) ) {
+				$results[] = false;
+			}
+			$results[] = true;
 			break 2;
 		}
 	}
+
+	if ( in_array( false, $results, true ) ) {
+		return false;
+	}
+	return count( $results );
+
 }
 
 /**
@@ -558,13 +619,16 @@ function _get_cron_array() {
  * Updates the CRON option with the new CRON array.
  *
  * @since 2.1.0
+ * @since 5.0.0 Return value modified to outcome of {@see update_option}.
+ *
  * @access private
  *
  * @param array $cron Cron info array from _get_cron_array().
+ * @return bool True if cron array updated, false on failure.
  */
 function _set_cron_array( $cron ) {
 	$cron['version'] = 2;
-	update_option( 'cron', $cron );
+	return update_option( 'cron', $cron );
 }
 
 /**
diff --git tests/phpunit/tests/cron.php tests/phpunit/tests/cron.php
index 8caab773d9..1c8ec97a1a 100644
--- tests/phpunit/tests/cron.php
+++ tests/phpunit/tests/cron.php
@@ -29,7 +29,8 @@ class Tests_Cron extends WP_UnitTestCase {
 		$hook      = __FUNCTION__;
 		$timestamp = strtotime( '+1 hour' );
 
-		wp_schedule_single_event( $timestamp, $hook );
+		$scheduled = wp_schedule_single_event( $timestamp, $hook );
+		$this->assertTrue( $scheduled );
 		$this->assertEquals( $timestamp, wp_next_scheduled( $hook ) );
 
 		// it's a non recurring event
@@ -43,7 +44,8 @@ class Tests_Cron extends WP_UnitTestCase {
 		$timestamp = strtotime( '+1 hour' );
 		$args      = array( 'foo' );
 
-		wp_schedule_single_event( $timestamp, $hook, $args );
+		$scheduled = wp_schedule_single_event( $timestamp, $hook, $args );
+		$this->assertTrue( $scheduled );
 		// this returns the timestamp only if we provide matching args
 		$this->assertEquals( $timestamp, wp_next_scheduled( $hook, $args ) );
 		// these don't match so return nothing
@@ -60,7 +62,8 @@ class Tests_Cron extends WP_UnitTestCase {
 		$recur     = 'hourly';
 		$timestamp = strtotime( '+1 hour' );
 
-		wp_schedule_event( $timestamp, $recur, $hook );
+		$scheduled = wp_schedule_event( $timestamp, $recur, $hook );
+		$this->assertTrue( $scheduled );
 		// it's scheduled for the right time
 		$this->assertEquals( $timestamp, wp_next_scheduled( $hook ) );
 		// it's a recurring event
@@ -74,7 +77,8 @@ class Tests_Cron extends WP_UnitTestCase {
 		$recur     = 'hourly';
 		$args      = array( 'foo' );
 
-		wp_schedule_event( $timestamp, 'hourly', $hook, $args );
+		$scheduled = wp_schedule_event( $timestamp, 'hourly', $hook, $args );
+		$this->assertTrue( $scheduled );
 		// this returns the timestamp only if we provide matching args
 		$this->assertEquals( $timestamp, wp_next_scheduled( $hook, $args ) );
 		// these don't match so return nothing
@@ -94,7 +98,8 @@ class Tests_Cron extends WP_UnitTestCase {
 		$this->assertEquals( $timestamp, wp_next_scheduled( $hook ) );
 
 		// now unschedule it and make sure it's gone
-		wp_unschedule_event( $timestamp, $hook );
+		$unscheduled = wp_unschedule_event( $timestamp, $hook );
+		$this->assertTrue( $unscheduled );
 		$this->assertEquals( false, wp_next_scheduled( $hook ) );
 	}
 
@@ -113,7 +118,8 @@ class Tests_Cron extends WP_UnitTestCase {
 		$this->assertTrue( wp_next_scheduled( $hook, $args ) > 0 );
 
 		// clear the schedule for the no args events and make sure it's gone
-		wp_clear_scheduled_hook( $hook );
+		$hook_unscheduled = wp_clear_scheduled_hook( $hook );
+		$this->assertSame( 2, $hook_unscheduled );
 		$this->assertFalse( wp_next_scheduled( $hook ) );
 		// the args events should still be there
 		$this->assertTrue( wp_next_scheduled( $hook, $args ) > 0 );
@@ -124,6 +130,18 @@ class Tests_Cron extends WP_UnitTestCase {
 		$this->assertFalse( wp_next_scheduled( $hook, $args ) );
 	}
 
+	function test_clear_undefined_schedule() {
+		$hook = __FUNCTION__;
+		$args = array( 'arg1' );
+
+		wp_schedule_single_event( strtotime( '+1 hour' ), $hook, $args );
+		wp_schedule_single_event( strtotime( '+2 hour' ), $hook, $args );
+
+		// clear the schedule for no args events and ensure no events are cleared.
+		$hook_unscheduled = wp_clear_scheduled_hook( $hook );
+		$this->assertSame( 0, $hook_unscheduled );
+	}
+
 	function test_clear_schedule_multiple_args() {
 		$hook = __FUNCTION__;
 		$args = array( 'arg1', 'arg2' );
@@ -206,7 +224,8 @@ class Tests_Cron extends WP_UnitTestCase {
 		$this->assertTrue( wp_next_scheduled( $hook, $args ) > 0 );
 
 		// clear the schedule and make sure it's gone.
-		wp_unschedule_hook( $hook );
+		$unschedule_hook = wp_unschedule_hook( $hook );
+		$this->assertSame( 4, $unschedule_hook );
 		$this->assertFalse( wp_next_scheduled( $hook ) );
 	}
 
@@ -221,9 +240,9 @@ class Tests_Cron extends WP_UnitTestCase {
 		$ts2  = strtotime( '+3 minutes' );
 
 		// first one works
-		wp_schedule_single_event( $ts1, $hook, $args );
+		$this->assertTrue( wp_schedule_single_event( $ts1, $hook, $args ) );
 		// second one is ignored
-		wp_schedule_single_event( $ts2, $hook, $args );
+		$this->assertFalse( wp_schedule_single_event( $ts2, $hook, $args ) );
 
 		// the next event should be at +5 minutes, not +3
 		$this->assertEquals( $ts1, wp_next_scheduled( $hook, $args ) );
@@ -240,9 +259,9 @@ class Tests_Cron extends WP_UnitTestCase {
 		$ts2  = strtotime( '+3 minutes' );
 
 		// first one works
-		wp_schedule_single_event( $ts1, $hook, $args );
+		$this->assertTrue( wp_schedule_single_event( $ts1, $hook, $args ) );
 		// second works too
-		wp_schedule_single_event( $ts2, $hook, $args );
+		$this->assertTrue( wp_schedule_single_event( $ts2, $hook, $args ) );
 
 		// the next event should be at +3 minutes, even though that one was scheduled second
 		$this->assertEquals( $ts2, wp_next_scheduled( $hook, $args ) );
@@ -259,9 +278,9 @@ class Tests_Cron extends WP_UnitTestCase {
 		$ts2  = strtotime( '+30 minutes' );
 
 		// first one works
-		wp_schedule_single_event( $ts1, $hook, $args );
+		$this->assertTrue( wp_schedule_single_event( $ts1, $hook, $args ) );
 		// second works too
-		wp_schedule_single_event( $ts2, $hook, $args );
+		$this->assertTrue( wp_schedule_single_event( $ts2, $hook, $args ) );
 
 		// the next event should be at +3 minutes
 		$this->assertEquals( $ts1, wp_next_scheduled( $hook, $args ) );