diff --git a/src/Standards/Typo3Update/Sniffs/LegacyClassnames/ClassnameCheckerTrait.php b/src/Standards/Typo3Update/Sniffs/LegacyClassnames/ClassnameCheckerTrait.php
index f3c7010..a4bddde 100644
--- a/src/Standards/Typo3Update/Sniffs/LegacyClassnames/ClassnameCheckerTrait.php
+++ b/src/Standards/Typo3Update/Sniffs/LegacyClassnames/ClassnameCheckerTrait.php
@@ -33,6 +33,16 @@ trait ClassnameCheckerTrait
      */
     private $legacyClassnames = [];
 
+    /**
+     * A list of extension names that might contain legacy class names.
+     * Used to check clas names for warnings.
+     *
+     * @var array<string>
+     */
+    private $legacyExtensions = [
+        'Extbase',
+    ];
+
     /**
      * @param string $mappingFile File containing php array for mapping.
      */
@@ -59,6 +69,31 @@ trait ClassnameCheckerTrait
         return isset($this->legacyClassnames[strtolower($classname)]);
     }
 
+    /**
+     * Guesses whether the given classname is legacy.  Will not check
+     * isLegacyClassname
+     *
+     * @param string $classname
+     * @return bool
+     */
+    private function isMaybeLegacyClassname($classname)
+    {
+        if (strpos($classname, 'Tx_') === false) {
+            return false;
+        }
+
+        $extensionName = call_user_func(function ($classname) {
+            $nameParts = explode('_', $classname);
+            return $nameParts[1];
+        }, $classname);
+
+        if (!in_array($extensionName, $this->legacyExtensions)) {
+            return false;
+        }
+
+        return true;
+    }
+
     /**
      * @param string $classname
      * @return string
@@ -78,6 +113,8 @@ trait ClassnameCheckerTrait
      */
     public function addFixableError(PhpcsFile $phpcsFile, $classnamePosition, $classname)
     {
+        $this->addMaybeWarning($phpcsFile, $classnamePosition, $classname);
+
         if ($this->isLegacyClassname($classname) === false) {
             return;
         }
@@ -97,6 +134,27 @@ trait ClassnameCheckerTrait
         }
     }
 
+    /**
+     * Add an warning if given $classname is maybe legacy.
+     *
+     * @param PhpcsFile $phpcsFile
+     * @param int $classnamePosition
+     * @param string $classname
+     */
+    private function addMaybeWarning(PhpcsFile $phpcsFile, $classnamePosition, $classname)
+    {
+        if ($this->isLegacyClassname($classname) || $this->isMaybeLegacyClassname($classname) === false) {
+            return;
+        }
+
+        $phpcsFile->addWarning(
+            'Legacy classes are not allowed; found %s that might be a legacy class that does not exist anymore',
+            $classnamePosition,
+            'mightBeLegacyClassname',
+            [$classname]
+        );
+    }
+
     /**
      * String to use for replacing / fixing the token.
      * Default is class name itself, can be overwritten in sniff for special behaviour.
diff --git a/src/Standards/Typo3Update/Sniffs/LegacyClassnames/InstanceofSniff.php b/src/Standards/Typo3Update/Sniffs/LegacyClassnames/InstanceofSniff.php
new file mode 100644
index 0000000..05cf416
--- /dev/null
+++ b/src/Standards/Typo3Update/Sniffs/LegacyClassnames/InstanceofSniff.php
@@ -0,0 +1,61 @@
+<?php
+
+/*
+ * Copyright (C) 2017  Daniel Siepmann <coding@daniel-siepmann.de>
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+ * 02110-1301, USA.
+ */
+
+/**
+ * Detect and migrate extend and implement of old legacy classnames.
+ */
+class Typo3Update_Sniffs_LegacyClassnames_InstanceofSniff implements PHP_CodeSniffer_Sniff
+{
+    use \Typo3Update\Sniffs\LegacyClassnames\ClassnameCheckerTrait;
+
+    /**
+     * Returns the token types that this sniff is interested in.
+     *
+     * @return array<int>
+     */
+    public function register()
+    {
+        return [
+            T_INSTANCEOF,
+        ];
+    }
+
+    /**
+     * Processes the tokens that this sniff is interested in.
+     *
+     * @param PHP_CodeSniffer_File $phpcsFile The file where the token was found.
+     * @param int                  $stackPtr  The position in the stack where
+     *                                        the token was found.
+     *
+     * @return void
+     */
+    public function process(PHP_CodeSniffer_File $phpcsFile, $stackPtr)
+    {
+        $tokens = $phpcsFile->getTokens();
+        $classnamePosition = $phpcsFile->findNext(T_STRING, $stackPtr);
+        if ($classnamePosition === false) {
+            return;
+        }
+        $classname = $tokens[$classnamePosition]['content'];
+
+        $this->addFixableError($phpcsFile, $classnamePosition, $classname);
+    }
+}