diff --git a/core/lib/Drupal/Core/Path/AliasManager.php b/core/lib/Drupal/Core/Path/AliasManager.php
index 5b43bf6..8c366f1 100644
--- a/core/lib/Drupal/Core/Path/AliasManager.php
+++ b/core/lib/Drupal/Core/Path/AliasManager.php
@@ -199,7 +199,7 @@ public function getAliasByPath($path, $langcode = NULL) {
     // Check the path whitelist, if the top-level part before the first /
     // is not in the list, then there is no need to do anything further,
     // it is not in the database.
-    if ($path === '/' || !$this->whitelist->get('/' . strtok(ltrim($path, '/'), '/'))) {
+    if ($path === '/' || !$this->whitelist->get(strtok(trim($path, '/'), '/'))) {
       return $path;
     }
 
diff --git a/core/lib/Drupal/Core/Path/AliasWhitelist.php b/core/lib/Drupal/Core/Path/AliasWhitelist.php
index 38ace76..fc26b77 100644
--- a/core/lib/Drupal/Core/Path/AliasWhitelist.php
+++ b/core/lib/Drupal/Core/Path/AliasWhitelist.php
@@ -107,7 +107,7 @@ public function get($offset) {
    * {@inheritdoc}
    */
   public function resolveCacheMiss($root) {
-    $exists = $this->aliasStorage->pathHasMatchingAlias($root);
+    $exists = $this->aliasStorage->pathHasMatchingAlias('/' . $root);
     $this->storage[$root] = $exists;
     $this->persist($root);
     if ($exists) {
diff --git a/core/modules/block/src/Tests/BlockTest.php b/core/modules/block/src/Tests/BlockTest.php
index d6e0999..8c65bc3 100644
--- a/core/modules/block/src/Tests/BlockTest.php
+++ b/core/modules/block/src/Tests/BlockTest.php
@@ -35,7 +35,7 @@ function testBlockVisibility() {
     );
     // Set the block to be hidden on any user path, and to be shown only to
     // authenticated users.
-    $edit['visibility[request_path][pages]'] = 'user*';
+    $edit['visibility[request_path][pages]'] = '/user*';
     $edit['visibility[request_path][negate]'] = TRUE;
     $edit['visibility[user_role][roles][' . RoleInterface::AUTHENTICATED_ID . ']'] = TRUE;
     $this->drupalGet('admin/structure/block/add/' . $block_name . '/' . $default_theme);
diff --git a/core/modules/page_cache/src/Tests/PageCacheTagsIntegrationTest.php b/core/modules/page_cache/src/Tests/PageCacheTagsIntegrationTest.php
index 95bd739..d8dec6a 100644
--- a/core/modules/page_cache/src/Tests/PageCacheTagsIntegrationTest.php
+++ b/core/modules/page_cache/src/Tests/PageCacheTagsIntegrationTest.php
@@ -64,7 +64,7 @@ function testPageCacheTags() {
     $block = $this->drupalPlaceBlock('views_block:comments_recent-block_1', array(
       'visibility' => array(
         'request_path' => array(
-          'pages' => 'node/' . $node_2->id(),
+          'pages' => '/node/' . $node_2->id(),
         ),
       ),
     ));
diff --git a/core/modules/path/src/Form/PathFormBase.php b/core/modules/path/src/Form/PathFormBase.php
index 6618dd9..be16b42 100644
--- a/core/modules/path/src/Form/PathFormBase.php
+++ b/core/modules/path/src/Form/PathFormBase.php
@@ -13,6 +13,7 @@
 use Drupal\Core\Path\AliasManagerInterface;
 use Drupal\Core\Path\AliasStorageInterface;
 use Drupal\Core\Path\PathValidatorInterface;
+use Drupal\Core\Routing\RequestContext;
 use Symfony\Component\DependencyInjection\ContainerInterface;
 
 /**
@@ -49,6 +50,13 @@
   protected $pathValidator;
 
   /**
+   * The request context.
+   *
+   * @var \Drupal\Core\Routing\RequestContext
+   */
+  protected $requestContext;
+
+  /**
    * Constructs a new PathController.
    *
    * @param \Drupal\Core\Path\AliasStorageInterface $alias_storage
@@ -57,11 +65,14 @@
    *   The path alias manager.
    * @param \Drupal\Core\Path\PathValidatorInterface $path_validator
    *   The path validator.
+   * @param \Drupal\Core\Routing\RequestContext $request_context
+   *   The request context.
    */
-  public function __construct(AliasStorageInterface $alias_storage, AliasManagerInterface $alias_manager, PathValidatorInterface $path_validator) {
+  public function __construct(AliasStorageInterface $alias_storage, AliasManagerInterface $alias_manager, PathValidatorInterface $path_validator, RequestContext $request_context) {
     $this->aliasStorage = $alias_storage;
     $this->aliasManager = $alias_manager;
     $this->pathValidator = $path_validator;
+    $this->requestContext = $request_context;
   }
 
   /**
@@ -71,7 +82,8 @@ public static function create(ContainerInterface $container) {
     return new static(
       $container->get('path.alias_storage'),
       $container->get('path.alias_manager'),
-      $container->get('path.validator')
+      $container->get('path.validator'),
+      $container->get('router.request_context')
     );
   }
 
@@ -94,8 +106,8 @@ public function buildForm(array $form, FormStateInterface $form_state, $pid = NU
       '#default_value' => $this->path['source'],
       '#maxlength' => 255,
       '#size' => 45,
-      '#description' => $this->t('Specify the existing path you wish to alias. For example: node/28, forum/1, taxonomy/term/1.'),
-      '#field_prefix' => $this->url('<none>', [], ['absolute' => TRUE]),
+      '#description' => $this->t('Specify the existing path you wish to alias. For example: /node/28, /forum/1, /taxonomy/term/1.'),
+      '#field_prefix' => $this->requestContext->getCompleteBaseUrl(),
       '#required' => TRUE,
     );
     $form['alias'] = array(
@@ -104,8 +116,8 @@ public function buildForm(array $form, FormStateInterface $form_state, $pid = NU
       '#default_value' => $this->path['alias'],
       '#maxlength' => 255,
       '#size' => 45,
-      '#description' => $this->t('Specify an alternative path by which this data can be accessed. For example, type "about" when writing an about page. Use a relative path.'),
-      '#field_prefix' => $this->url('<none>', [], ['absolute' => TRUE]),
+      '#description' => $this->t('Specify an alternative path by which this data can be accessed. For example, type "/about" when writing an about page. Use a relative path with a slash in front..'),
+      '#field_prefix' => $this->requestContext->getCompleteBaseUrl(),
       '#required' => TRUE,
     );
 
diff --git a/core/modules/path/src/Plugin/Field/FieldWidget/PathWidget.php b/core/modules/path/src/Plugin/Field/FieldWidget/PathWidget.php
index 957484c..07f1712 100644
--- a/core/modules/path/src/Plugin/Field/FieldWidget/PathWidget.php
+++ b/core/modules/path/src/Plugin/Field/FieldWidget/PathWidget.php
@@ -96,7 +96,7 @@ public static function validateFormElement(array &$element, FormStateInterface $
       }
     }
 
-    if ($alias[0] !== '/') {
+    if ($alias && $alias[0] !== '/') {
       $form_state->setError($element, t('The alias needs to start with a slash.'));
     }
   }
diff --git a/core/modules/system/src/PathBasedBreadcrumbBuilder.php b/core/modules/system/src/PathBasedBreadcrumbBuilder.php
index 45418a7b..2d6a5eb 100644
--- a/core/modules/system/src/PathBasedBreadcrumbBuilder.php
+++ b/core/modules/system/src/PathBasedBreadcrumbBuilder.php
@@ -138,11 +138,11 @@ public function build(RouteMatchInterface $route_match) {
     $exclude[$front] = TRUE;
     // /user is just a redirect, so skip it.
     // @todo Find a better way to deal with /user.
-    $exclude['user'] = TRUE;
+    $exclude['/user'] = TRUE;
     while (count($path_elements) > 1) {
       array_pop($path_elements);
       // Copy the path elements for up-casting.
-      $route_request = $this->getRequestForPath(implode('/', $path_elements), $exclude);
+      $route_request = $this->getRequestForPath('/' . implode('/', $path_elements), $exclude);
       if ($route_request) {
         $route_match = RouteMatch::createFromRequest($route_request);
         $access = $this->accessManager->check($route_match, $this->currentUser);
@@ -172,7 +172,7 @@ public function build(RouteMatchInterface $route_match) {
    * Matches a path in the router.
    *
    * @param string $path
-   *   The request path.
+   *   The request path with a leading slash.
    * @param array $exclude
    *   An array of paths or system paths to skip.
    *
@@ -185,7 +185,7 @@ protected function getRequestForPath($path, array $exclude) {
     }
     // @todo Use the RequestHelper once https://www.drupal.org/node/2090293 is
     //   fixed.
-    $request = Request::create($this->context->getCompleteBaseUrl() . '/' . $path);
+    $request = Request::create($this->context->getCompleteBaseUrl() . $path);
     // Performance optimization: set a short accept header to reduce overhead in
     // AcceptHeaderMatcher when matching the request.
     $request->headers->set('Accept', 'text/html');
@@ -195,7 +195,7 @@ protected function getRequestForPath($path, array $exclude) {
       // This resolves to the front page, which we already add.
       return NULL;
     }
-    $this->currentPath->setPath('/' . $processed, $request);
+    $this->currentPath->setPath($processed, $request);
     // Attempt to match this path to provide a fully built request.
     try {
       $request->attributes->add($this->router->matchRequest($request));
diff --git a/core/modules/system/src/Tests/Routing/ContentNegotiationRoutingTest.php b/core/modules/system/src/Tests/Routing/ContentNegotiationRoutingTest.php
index e9430b0..53f1efb 100644
--- a/core/modules/system/src/Tests/Routing/ContentNegotiationRoutingTest.php
+++ b/core/modules/system/src/Tests/Routing/ContentNegotiationRoutingTest.php
@@ -56,10 +56,10 @@ function testContentRouting() {
     /** @var \Drupal\Core\Path\AliasStorageInterface $path_alias_storage */
     $path_alias_storage = $this->container->get('path.alias_storage');
     // Alias with extension pointing to no extension/constant content-type.
-    $path_alias_storage->save('conneg/html', 'alias.html');
+    $path_alias_storage->save('/conneg/html', '/alias.html');
 
     // Alias with extension pointing to dynamic extension/linked content-type.
-    $path_alias_storage->save('conneg/html?_format=json', 'alias.json');
+    $path_alias_storage->save('/conneg/html?_format=json', '/alias.json');
 
     $tests = [
       // ['path', 'accept', 'content-type'],
@@ -102,7 +102,7 @@ function testContentRouting() {
       $accept_header = $test[1];
       $content_type = $test[2];
       $message = "Testing path:$path Accept:$accept_header Content-type:$content_type";
-      $request = Request::create($path);
+      $request = Request::create('/' . $path);
       if ($accept_header) {
         $request->headers->set('Accept', $accept_header);
       }
@@ -141,7 +141,7 @@ public function testFullNegotiation() {
       $accept_header = $test[1];
       $content_type = $test[2];
       $message = "Testing path:$path Accept:$accept_header Content-type:$content_type";
-      $request = Request::create($path);
+      $request = Request::create('/' . $path);
       $request->headers->set('Accept', $accept_header);
 
       /** @var \Symfony\Component\HttpKernel\HttpKernelInterface $kernel */
diff --git a/core/modules/system/src/Tests/System/AccessDeniedTest.php b/core/modules/system/src/Tests/System/AccessDeniedTest.php
index 17cb9d5..ebc5eea 100644
--- a/core/modules/system/src/Tests/System/AccessDeniedTest.php
+++ b/core/modules/system/src/Tests/System/AccessDeniedTest.php
@@ -44,7 +44,7 @@ function testAccessDenied() {
     // Use a custom 403 page.
     $this->drupalLogin($this->adminUser);
     $edit = [
-      'site_403' => 'user/' . $this->adminUser->id(),
+      'site_403' => '/user/' . $this->adminUser->id(),
     ];
     $this->drupalPostForm('admin/config/system/site-information', $edit, t('Save configuration'));
 
@@ -73,7 +73,7 @@ function testAccessDenied() {
 
     // Log back in, set the custom 403 page to /user/login and remove the block
     $this->drupalLogin($this->adminUser);
-    $this->config('system.site')->set('page.403', 'user/login')->save();
+    $this->config('system.site')->set('page.403', '/user/login')->save();
     $edit = [
       'region' => -1,
     ];
diff --git a/core/tests/Drupal/Tests/Core/Path/PathMatcherTest.php b/core/tests/Drupal/Tests/Core/Path/PathMatcherTest.php
index 1d1aaf7..e60f8f5 100644
--- a/core/tests/Drupal/Tests/Core/Path/PathMatcherTest.php
+++ b/core/tests/Drupal/Tests/Core/Path/PathMatcherTest.php
@@ -121,43 +121,43 @@ public function getMatchPathData() {
       ),
       array(
         // Multiple paths with the \n delimiter.
-        "node/*\nnode/*/edit",
+        "/node/*\n/node/*/edit",
         array(
-          'node/1' => TRUE,
-          'node/view' => TRUE,
-          'node/32/edit' => TRUE,
-          'node/delete/edit' => TRUE,
-          'node/50/delete' => TRUE,
-          'test/example' => FALSE,
+          '/node/1' => TRUE,
+          '/node/view' => TRUE,
+          '/node/32/edit' => TRUE,
+          '/node/delete/edit' => TRUE,
+          '/node/50/delete' => TRUE,
+          '/test/example' => FALSE,
         ),
       ),
       array(
         // Multiple paths with the \r delimiter.
-        "user/*\rexample/*",
+        "/user/*\r/example/*",
         array(
-          'user/1' => TRUE,
-          'example/1' => TRUE,
-          'user/1/example/1' => TRUE,
-          'user/example' => TRUE,
-          'test/example' => FALSE,
-          'user' => FALSE,
-          'example' => FALSE,
+          '/user/1' => TRUE,
+          '/example/1' => TRUE,
+          '/user/1/example/1' => TRUE,
+          '/user/example' => TRUE,
+          '/test/example' => FALSE,
+          '/user' => FALSE,
+          '/example' => FALSE,
         ),
       ),
       array(
         // Multiple paths with the \r\n delimiter.
-        "test\r\n<front>",
+        "/test\r\n<front>",
         array(
-          'test' => TRUE,
-          'dummy' => TRUE,
-          'example' => FALSE,
+          '/test' => TRUE,
+          '/dummy' => TRUE,
+          '/example' => FALSE,
         ),
       ),
       array(
         // Test existing regular expressions (should be escaped).
         '[^/]+?/[0-9]',
         array(
-          'test/1' => FALSE,
+          '/test/1' => FALSE,
           '[^/]+?/[0-9]' => TRUE,
         ),
       ),
diff --git a/core/tests/Drupal/Tests/Core/PathProcessor/PathProcessorTest.php b/core/tests/Drupal/Tests/Core/PathProcessor/PathProcessorTest.php
index 4560881..79b49a2 100644
--- a/core/tests/Drupal/Tests/Core/PathProcessor/PathProcessorTest.php
+++ b/core/tests/Drupal/Tests/Core/PathProcessor/PathProcessorTest.php
@@ -103,11 +103,11 @@ function testProcessInbound() {
 
     $system_path_map = array(
       // Set up one proper alias that can be resolved to a system path.
-      array('foo', NULL, 'user/1'),
+      array('/foo', NULL, '/user/1'),
       // Passing in anything else should return the same string.
-      array('fr/foo', NULL, 'fr/foo'),
-      array('fr', NULL, 'fr'),
-      array('user/login', NULL, 'user/login'),
+      array('/fr/foo', NULL, '/fr/foo'),
+      array('/fr', NULL, '/fr'),
+      array('/user/login', NULL, '/user/login'),
     );
 
     $alias_manager->expects($this->any())
@@ -169,16 +169,16 @@ function testProcessInbound() {
     }
 
     // Test resolving the French homepage using the incorrect processor order.
-    $test_path = 'fr';
+    $test_path = '/fr';
     $request = Request::create($test_path);
     $processed = $processor_manager->processInbound($test_path, $request);
-    $this->assertEquals('', $processed, 'Processing in the incorrect order fails to resolve the system path from the empty path');
+    $this->assertEquals('/', $processed, 'Processing in the incorrect order fails to resolve the system path from the empty path');
 
     // Test resolving an existing alias using the incorrect processor order.
-    $test_path = 'fr/foo';
+    $test_path = '/fr/foo';
     $request = Request::create($test_path);
     $processed = $processor_manager->processInbound($test_path, $request);
-    $this->assertEquals('foo', $processed, 'Processing in the incorrect order fails to resolve the system path from an alias');
+    $this->assertEquals('/foo', $processed, 'Processing in the incorrect order fails to resolve the system path from an alias');
 
     // Now create a new processor manager and add the processors, this time in
     // the correct order.
@@ -194,15 +194,15 @@ function testProcessInbound() {
     }
 
     // Test resolving the French homepage using the correct processor order.
-    $test_path = 'fr';
+    $test_path = '/fr';
     $request = Request::create($test_path);
     $processed = $processor_manager->processInbound($test_path, $request);
-    $this->assertEquals('user/login', $processed, 'Processing in the correct order resolves the system path from the empty path.');
+    $this->assertEquals('/user/login', $processed, 'Processing in the correct order resolves the system path from the empty path.');
 
     // Test resolving an existing alias using the correct processor order.
-    $test_path = 'fr/foo';
+    $test_path = '/fr/foo';
     $request = Request::create($test_path);
     $processed = $processor_manager->processInbound($test_path, $request);
-    $this->assertEquals('user/1', $processed, 'Processing in the correct order resolves the system path from an alias.');
+    $this->assertEquals('/user/1', $processed, 'Processing in the correct order resolves the system path from an alias.');
   }
 }