diff --git a/src/Index/AbstractAggregateIndex.php b/src/Index/AbstractAggregateIndex.php
index 5377c3a..33b86e5 100644
--- a/src/Index/AbstractAggregateIndex.php
+++ b/src/Index/AbstractAggregateIndex.php
@@ -147,4 +147,14 @@ abstract class AbstractAggregateIndex implements ReadableIndex
         }
         return $refs;
     }
+
+    /**
+     * Wipe all indexes for a reindex
+     */
+    public function wipe()
+    {
+        foreach ($this->getIndexes() as $index) {
+            $index->wipe();
+        }
+    }
 }
diff --git a/src/Index/Index.php b/src/Index/Index.php
index 9cb975e..2755ad7 100644
--- a/src/Index/Index.php
+++ b/src/Index/Index.php
@@ -222,4 +222,15 @@ class Index implements ReadableIndex, \Serializable
             'staticComplete' => $this->staticComplete
         ]);
     }
+
+    /**
+     * Clear indexed references and definitions
+     */
+    public function wipe()
+    {
+        $this->definitions = [];
+        $this->references = [];
+        $this->complete = false;
+        $this->staticComplete = false;
+    }
 }
diff --git a/src/Indexer.php b/src/Indexer.php
index e411054..253c7d7 100644
--- a/src/Indexer.php
+++ b/src/Indexer.php
@@ -68,6 +68,16 @@ class Indexer
      */
     private $composerJson;
 
+    /**
+     * @var bool
+     */
+    private $hasCancellationSignal;
+
+    /**
+     * @var bool
+     */
+    private $isIndexing;
+
     /**
      * @param FilesFinder       $filesFinder
      * @param string            $rootPath
@@ -101,6 +111,8 @@ class Indexer
         $this->options = $options;
         $this->composerLock = $composerLock;
         $this->composerJson = $composerJson;
+        $this->hasCancellationSignal = false;
+        $this->isIndexing = false;
     }
 
     /**
@@ -118,6 +130,7 @@ class Indexer
             $count = count($uris);
             $startTime = microtime(true);
             $this->client->window->logMessage(MessageType::INFO, "$count files total");
+            $this->isIndexing = true;
 
             /** @var string[] */
             $source = [];
@@ -195,6 +208,7 @@ class Indexer
                 }
             }
 
+            $this->isIndexing = false;
             $duration = (int)(microtime(true) - $startTime);
             $mem = (int)(memory_get_usage(true) / (1024 * 1024));
             $this->client->window->logMessage(
@@ -204,6 +218,34 @@ class Indexer
         });
     }
 
+    /**
+     * Return current indexing state
+     *
+     * @return bool
+     */
+    public function isIndexing(): bool
+    {
+        return $this->isIndexing;
+    }
+
+    /**
+     * Cancel all running indexing processes
+     *
+     * @return Promise
+     */
+    public function cancel(): Promise
+    {
+        return coroutine(function () {
+            $this->hasCancellationSignal = true;
+
+            while ($this->isIndexing()) {
+                yield timeout();
+            }
+
+            $this->hasCancellationSignal = false;
+        });
+    }
+
     /**
      * @param array $files
      * @return Promise
@@ -212,6 +254,10 @@ class Indexer
     {
         return coroutine(function () use ($files) {
             foreach ($files as $i => $uri) {
+                if ($this->hasCancellationSignal) {
+                    return;
+                }
+
                 // Skip open documents
                 if ($this->documentLoader->isOpen($uri)) {
                     continue;
diff --git a/src/LanguageServer.php b/src/LanguageServer.php
index 00822d7..29cfc4c 100644
--- a/src/LanguageServer.php
+++ b/src/LanguageServer.php
@@ -329,6 +329,7 @@ class LanguageServer extends AdvancedJsonRpc\Dispatcher
                     $dependenciesIndex,
                     $sourceIndex,
                     $options,
+                    $indexer,
                     $this->composerLock,
                     $this->documentLoader,
                     $this->composerJson
diff --git a/src/Server/Workspace.php b/src/Server/Workspace.php
index 548197b..8eee501 100644
--- a/src/Server/Workspace.php
+++ b/src/Server/Workspace.php
@@ -3,11 +3,12 @@ declare(strict_types = 1);
 
 namespace LanguageServer\Server;
 
-use LanguageServer\{LanguageClient, PhpDocumentLoader};
+use LanguageServer\{Indexer, LanguageClient, Options, PhpDocumentLoader};
 use LanguageServer\Index\{ProjectIndex, DependenciesIndex, Index};
 use LanguageServer\Protocol\{
     FileChangeType,
     FileEvent,
+    MessageType,
     SymbolInformation,
     SymbolDescriptor,
     ReferenceInformation,
@@ -45,6 +46,16 @@ class Workspace
      */
     private $sourceIndex;
 
+    /**
+     * @var Options
+     */
+    private $options;
+
+    /**
+     * @var Indexer
+     */
+    private $indexer;
+
     /**
      * @var \stdClass
      */
@@ -60,11 +71,22 @@ class Workspace
      * @param ProjectIndex      $projectIndex      Index that is used to wait for full index completeness
      * @param DependenciesIndex $dependenciesIndex Index that is used on a workspace/xreferences request
      * @param DependenciesIndex $sourceIndex       Index that is used on a workspace/xreferences request
+     * @param Options           $options           Initialization options that are used on a workspace/didChangeConfiguration
+     * @param Indexer           $indexer
      * @param \stdClass         $composerLock      The parsed composer.lock of the project, if any
      * @param PhpDocumentLoader $documentLoader    PhpDocumentLoader instance to load documents
      */
-    public function __construct(LanguageClient $client, ProjectIndex $projectIndex, DependenciesIndex $dependenciesIndex, Index $sourceIndex, \stdClass $composerLock = null, PhpDocumentLoader $documentLoader, \stdClass $composerJson = null)
-    {
+    public function __construct(
+        LanguageClient $client,
+        ProjectIndex $projectIndex,
+        DependenciesIndex $dependenciesIndex,
+        Index $sourceIndex,
+        Options $options,
+        Indexer $indexer,
+        \stdClass $composerLock = null,
+        PhpDocumentLoader $documentLoader,
+        \stdClass $composerJson = null
+    ) {
         $this->client = $client;
         $this->sourceIndex = $sourceIndex;
         $this->projectIndex = $projectIndex;
@@ -72,10 +94,13 @@ class Workspace
         $this->composerLock = $composerLock;
         $this->documentLoader = $documentLoader;
         $this->composerJson = $composerJson;
+        $this->options = $options;
+        $this->indexer = $indexer;
     }
 
     /**
-     * The workspace symbol request is sent from the client to the server to list project-wide symbols matching the query string.
+     * The workspace symbol request is sent from the client to the server to list
+     * project-wide symbols matching the query string.
      *
      * @param string $query
      * @return Promise <SymbolInformation[]>
@@ -98,7 +123,8 @@ class Workspace
     }
 
     /**
-     * The watched files notification is sent from the client to the server when the client detects changes to files watched by the language client.
+     * The watched files notification is sent from the client to the server when
+     * the client detects changes to files watched by the language client.
      *
      * @param FileEvent[] $changes
      * @return void
@@ -113,7 +139,8 @@ class Workspace
     }
 
     /**
-     * The workspace references request is sent from the client to the server to locate project-wide references to a symbol given its description / metadata.
+     * The workspace references request is sent from the client to the server to
+     * locate project-wide references to a symbol given its description / metadata.
      *
      * @param SymbolDescriptor $query Partial metadata about the symbol that is being searched for.
      * @param string[]         $files An optional list of files to restrict the search to.
@@ -174,4 +201,47 @@ class Workspace
         }
         return $dependencyReferences;
     }
+
+    /**
+     * A notification sent from the client to the server to signal the change of configuration settings.
+     *
+     * @param \stdClass $settings Settings as JSON object structure with php as primary key
+     * @return Promise
+     */
+    public function didChangeConfiguration(\stdClass $settings): Promise
+    {
+        xdebug_break();
+        return coroutine(function () use ($settings) {
+            try {
+                xdebug_break();
+                $mapper = new \JsonMapper();
+                $settings = $mapper->map($settings->php, new Options);
+
+                if ($this->options == $settings) {
+                    return;
+                }
+
+                // @TODO: get changed settings and apply them
+                // @TODO: check settings that affect the indexer
+
+                if ($this->indexer->isIndexing()) {
+                    yield $this->indexer->cancel();
+                }
+
+                $this->projectIndex->wipe();
+                $this->indexer->index();
+
+                $this->client->window->showMessage(
+                    MessageType::INFO,
+                    'Reindexing with new settings.'
+                );
+            } catch (\JsonMapper_Exception $exception) {
+                $this->client->window->showMessage(
+                    MessageType::ERROR,
+                    'Settings could not be applied. For more information see logs.'
+                );
+                $this->client->window->logMessage(MessageType::ERROR, $exception->getMessage());
+            }
+        });
+    }
 }
diff --git a/tests/Server/ServerTestCase.php b/tests/Server/ServerTestCase.php
index 45d949f..27d64e0 100644
--- a/tests/Server/ServerTestCase.php
+++ b/tests/Server/ServerTestCase.php
@@ -5,9 +5,7 @@ namespace LanguageServer\Tests\Server;
 
 use PHPUnit\Framework\TestCase;
 use LanguageServer\Tests\MockProtocolStream;
-use LanguageServer\{
-    Server, LanguageClient, PhpDocumentLoader, DefinitionResolver
-};
+use LanguageServer\{Options, Server, LanguageClient, PhpDocumentLoader, DefinitionResolver};
 use LanguageServer\Index\{ProjectIndex, DependenciesIndex, Index};
 use LanguageServer\ContentRetriever\FileSystemContentRetriever;
 use LanguageServer\Protocol\{Position, Location, Range};
@@ -46,6 +44,7 @@ abstract class ServerTestCase extends TestCase
 
     public function setUp()
     {
+        $options           = new Options();
         $sourceIndex       = new Index;
         $dependenciesIndex = new DependenciesIndex;
         $projectIndex      = new ProjectIndex($sourceIndex, $dependenciesIndex);
@@ -55,7 +54,7 @@ abstract class ServerTestCase extends TestCase
         $client               = new LanguageClient(new MockProtocolStream, new MockProtocolStream);
         $this->documentLoader = new PhpDocumentLoader(new FileSystemContentRetriever, $projectIndex, $definitionResolver);
         $this->textDocument   = new Server\TextDocument($this->documentLoader, $definitionResolver, $client, $projectIndex);
-        $this->workspace      = new Server\Workspace($client, $projectIndex, $dependenciesIndex, $sourceIndex, null, $this->documentLoader);
+        $this->workspace      = new Server\Workspace($client, $projectIndex, $dependenciesIndex, $sourceIndex, $options, null, $this->documentLoader);
 
         $globalSymbolsUri    = pathToUri(realpath(__DIR__ . '/../../fixtures/global_symbols.php'));
         $globalReferencesUri = pathToUri(realpath(__DIR__ . '/../../fixtures/global_references.php'));
diff --git a/tests/Server/Workspace/DidChangeConfigurationTest.php b/tests/Server/Workspace/DidChangeConfigurationTest.php
new file mode 100644
index 0000000..031852e
--- /dev/null
+++ b/tests/Server/Workspace/DidChangeConfigurationTest.php
@@ -0,0 +1,231 @@
+<?php
+/**
+ * Copyright (c) 2018 Jürgen Steitz
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+declare(strict_types=1);
+
+namespace LanguageServer\Tests\Server\Workspace;
+
+use Exception;
+use LanguageServer\Cache\FileSystemCache;
+use LanguageServer\ContentRetriever\FileSystemContentRetriever;
+use LanguageServer\DefinitionResolver;
+use LanguageServer\FilesFinder\FileSystemFilesFinder;
+use LanguageServer\Index\DependenciesIndex;
+use LanguageServer\Index\Index;
+use LanguageServer\Index\ProjectIndex;
+use LanguageServer\Indexer;
+use LanguageServer\LanguageClient;
+use LanguageServer\Options;
+use LanguageServer\PhpDocumentLoader;
+use LanguageServer\Protocol\Message;
+use LanguageServer\Protocol\MessageType;
+use LanguageServer\Server;
+use LanguageServer\Tests\MockProtocolStream;
+use LanguageServer\Tests\Server\ServerTestCase;
+use Sabre\Event\Promise;
+
+class DidChangeConfigurationTest extends ServerTestCase
+{
+    public function testFailsWithInvalidOptionsTypeOrFormat()
+    {
+        $promise = new Promise;
+        $sourceIndex = new Index;
+        $dependenciesIndex = new DependenciesIndex;
+        $projectIndex = new ProjectIndex($sourceIndex, $dependenciesIndex);
+        $projectIndex->setComplete();
+
+        $rootPath = realpath(__DIR__ . '/../../../fixtures/');
+        $filesFinder = new FileSystemFilesFinder;
+        $cache = new FileSystemCache;
+        $initialOptions = new Options;
+
+        $input = new MockProtocolStream;
+        $output = new MockProtocolStream;
+
+        $definitionResolver = new DefinitionResolver($projectIndex);
+        $client = new LanguageClient($input, $output);
+        $documentLoader = new PhpDocumentLoader(new FileSystemContentRetriever, $projectIndex, $definitionResolver);
+        $textDocument = new Server\TextDocument($documentLoader, $definitionResolver, $client, $projectIndex);
+        $indexer = new Indexer(
+            $filesFinder,
+            $rootPath,
+            $client,
+            $cache,
+            $dependenciesIndex,
+            $sourceIndex,
+            $documentLoader,
+            $initialOptions
+        );
+        $workspace = new Server\Workspace(
+            $client,
+            $projectIndex,
+            $dependenciesIndex,
+            $sourceIndex,
+            $initialOptions,
+            null,
+            $documentLoader
+        );
+
+        $output->on('message', function (Message $msg) use ($promise) {
+            if ($msg->body->method === 'window/showMessage' && $promise->state === Promise::PENDING) {
+                $hasMessage = strpos(
+                    $msg->body->params->message,
+                    'Settings could not be applied. For more information see logs.'
+                ) !== false;
+
+                if ($msg->body->params->type === MessageType::ERROR && $hasMessage) {
+                    $promise->fulfill(true);
+                }
+
+                if ($msg->body->params->type !== MessageType::ERROR) {
+                    $promise->reject(new Exception($msg->body->params->message));
+                }
+            }
+        });
+
+        $settings = new \stdClass();
+        $settings->php = new \stdClass();
+        $settings->php->fileTypes = 'not an array';
+
+        $workspace->didChangeConfiguration($settings);
+        $this->assertTrue($promise->wait());
+    }
+
+    public function testNoChangedOptions()
+    {
+        $promise = new Promise;
+        $sourceIndex = new Index;
+        $dependenciesIndex = new DependenciesIndex;
+        $projectIndex = new ProjectIndex($sourceIndex, $dependenciesIndex);
+        $projectIndex->setComplete();
+
+        $rootPath = realpath(__DIR__ . '/../../../fixtures/');
+        $filesFinder = new FileSystemFilesFinder;
+        $cache = new FileSystemCache;
+        $initialOptions = new Options;
+
+        $input = new MockProtocolStream;
+        $output = new MockProtocolStream;
+
+        $definitionResolver = new DefinitionResolver($projectIndex);
+        $client = new LanguageClient($input, $output);
+        $documentLoader = new PhpDocumentLoader(new FileSystemContentRetriever, $projectIndex, $definitionResolver);
+        $textDocument = new Server\TextDocument($documentLoader, $definitionResolver, $client, $projectIndex);
+        $indexer = new Indexer(
+            $filesFinder,
+            $rootPath,
+            $client,
+            $cache,
+            $dependenciesIndex,
+            $sourceIndex,
+            $documentLoader,
+            $initialOptions
+        );
+        $workspace = new Server\Workspace(
+            $client,
+            $projectIndex,
+            $dependenciesIndex,
+            $sourceIndex,
+            $initialOptions,
+            null,
+            $documentLoader
+        );
+
+        $output->on('message', function (Message $msg) use ($promise) {
+            $promise->reject(new Exception($msg->body->message));
+        });
+
+        $settings = new \stdClass();
+        $settings->php = new \stdClass();
+        $settings->php->fileTypes = ['.php'];
+
+        $this->expectException(\LogicException::class);
+        $workspace->didChangeConfiguration($settings);
+        $promise->wait();
+    }
+
+    public function testDetectsChangedOptions()
+    {
+        $promise = new Promise;
+        $sourceIndex = new Index;
+        $dependenciesIndex = new DependenciesIndex;
+        $projectIndex = new ProjectIndex($sourceIndex, $dependenciesIndex);
+        $projectIndex->setComplete();
+
+        $rootPath = realpath(__DIR__ . '/../../../fixtures/');
+        $filesFinder = new FileSystemFilesFinder;
+        $cache = new FileSystemCache;
+        $initialOptions = new Options;
+
+        $input = new MockProtocolStream;
+        $output = new MockProtocolStream;
+
+        $definitionResolver = new DefinitionResolver($projectIndex);
+        $client = new LanguageClient($input, $output);
+        $documentLoader = new PhpDocumentLoader(new FileSystemContentRetriever, $projectIndex, $definitionResolver);
+        $textDocument = new Server\TextDocument($documentLoader, $definitionResolver, $client, $projectIndex);
+        $indexer = new Indexer(
+            $filesFinder,
+            $rootPath,
+            $client,
+            $cache,
+            $dependenciesIndex,
+            $sourceIndex,
+            $documentLoader,
+            $initialOptions
+        );
+        $workspace = new Server\Workspace(
+            $client,
+            $projectIndex,
+            $dependenciesIndex,
+            $sourceIndex,
+            $initialOptions,
+            null,
+            $documentLoader
+        );
+
+        $output->on('message', function (Message $msg) use ($promise) {
+            if ($msg->body->method === 'window/showMessage' && $promise->state === Promise::PENDING) {
+                $hasMessage = strpos(
+                    $msg->body->params->message,
+                    'You must restart your editor for the changes to take effect.'
+                ) !== false;
+
+                if ($msg->body->params->type === MessageType::INFO && $hasMessage) {
+                    $promise->fulfill(true);
+                }
+
+                if ($msg->body->params->type === MessageType::ERROR) {
+                    $promise->reject(new Exception($msg->body->params->message));
+                }
+            }
+        });
+
+        $settings = new \stdClass();
+        $settings->php = new \stdClass();
+        $settings->php->fileTypes = ['.php', '.php5']; // default is only .php
+
+        $workspace->didChangeConfiguration($settings);
+        $this->assertTrue($promise->wait());
+    }
+}
diff --git a/tests/Server/Workspace/DidChangeWatchedFilesTest.php b/tests/Server/Workspace/DidChangeWatchedFilesTest.php
index 1074c58..d93985c 100644
--- a/tests/Server/Workspace/DidChangeWatchedFilesTest.php
+++ b/tests/Server/Workspace/DidChangeWatchedFilesTest.php
@@ -4,7 +4,7 @@ declare(strict_types = 1);
 namespace LanguageServer\Tests\Server\Workspace;
 
 use LanguageServer\ContentRetriever\FileSystemContentRetriever;
-use LanguageServer\{DefinitionResolver, LanguageClient, PhpDocumentLoader, Server};
+use LanguageServer\{DefinitionResolver, LanguageClient, Options, PhpDocumentLoader, Server};
 use LanguageServer\Index\{DependenciesIndex, Index, ProjectIndex};
 use LanguageServer\Protocol\{FileChangeType, FileEvent, Message};
 use LanguageServer\Tests\MockProtocolStream;
@@ -16,11 +16,12 @@ class DidChangeWatchedFilesTest extends ServerTestCase
 {
     public function testDeletingFileClearsAllDiagnostics()
     {
+        $options = new Options();
         $client = new LanguageClient(new MockProtocolStream(), $writer = new MockProtocolStream());
         $projectIndex = new ProjectIndex($sourceIndex = new Index(), $dependenciesIndex = new DependenciesIndex());
         $definitionResolver = new DefinitionResolver($projectIndex);
         $loader = new PhpDocumentLoader(new FileSystemContentRetriever(), $projectIndex, $definitionResolver);
-        $workspace = new Server\Workspace($client, $projectIndex, $dependenciesIndex, $sourceIndex, null, $loader, null);
+        $workspace = new Server\Workspace($client, $projectIndex, $dependenciesIndex, $sourceIndex, $options, null, $loader, null);
 
         $fileEvent = new FileEvent('my uri', FileChangeType::DELETED);