diff --git a/TODO b/TODO
index 650e1900..32fb1635 100644
--- a/TODO
+++ b/TODO
@@ -6,7 +6,6 @@ everything related to posts:
         - fav
         - score (see notes about scoring)
         - editing
-            - concurrency
             - relations
             - ability to loop video posts
             - post edit history (think
diff --git a/public_html/js/Presenters/PostPresenter.js b/public_html/js/Presenters/PostPresenter.js
index a9a4925a..d8561b07 100644
--- a/public_html/js/Presenters/PostPresenter.js
+++ b/public_html/js/Presenters/PostPresenter.js
@@ -168,6 +168,7 @@ App.Presenters.PostPresenter = function(
 	function editPost() {
 		var $form = $el.find('form');
 		var formData = {};
+		formData.seenEditTime = post.lastEditTime;
 
 		if (editPrivileges.canChangeContent && postContent) {
 			formData.content = postContent;
diff --git a/src/FormData/PostEditFormData.php b/src/FormData/PostEditFormData.php
index a91e4a03..98f2d4c7 100644
--- a/src/FormData/PostEditFormData.php
+++ b/src/FormData/PostEditFormData.php
@@ -9,6 +9,8 @@ class PostEditFormData implements \Szurubooru\IValidatable
 	public $source;
 	public $tags;
 
+	public $seenEditTime;
+
 	public function __construct($inputReader = null)
 	{
 		if ($inputReader !== null)
@@ -18,6 +20,7 @@ class PostEditFormData implements \Szurubooru\IValidatable
 			$this->safety = \Szurubooru\Helpers\EnumHelper::postSafetyFromString($inputReader->safety);
 			$this->source = $inputReader->source;
 			$this->tags = preg_split('/[\s+]/', $inputReader->tags);
+			$this->seenEditTime = $inputReader->seenEditTime;
 		}
 	}
 
diff --git a/src/Services/PostService.php b/src/Services/PostService.php
index 57f0ec55..491b593f 100644
--- a/src/Services/PostService.php
+++ b/src/Services/PostService.php
@@ -115,6 +115,10 @@ class PostService
 		$transactionFunc = function() use ($post, $formData)
 		{
 			$this->validator->validate($formData);
+
+			if ($post->getLastEditTime() !== $formData->seenEditTime)
+				throw new \DomainException('Someone has already edited this post in the meantime.');
+
 			$post->setLastEditTime($this->timeService->getCurrentTime());
 
 			if ($formData->content !== null)