If you're seeing pre-roll or in-video ads on YouTube.com or other websites, it may be the video player's fault.
Nearly all modern browsers now use the newer HTML5 video player for website content by default. Most websites have switched to using the HTML5 video player, too. HTML5 videos are lighter on resources and require less power to run, so they load faster and your laptop battery lasts longer. HTML5 is also far more secure than the older Adobe Flash technology it's replacing. AdBlock blocks ads in HTML5 videos and games anywhere they appear, in any browser.
Some websites still use the Adobe Flash player for video content. Flash games and embedded YouTube videos on those sites may have unblocked ads. Why? Flash network requests aren't passed through browser extensions. That means AdBlock, which is a browser extension, can't "see" Flash video. And that means browser-based ad blockers such as AdBlock can't block ads in Flash videos and games (with one exception; see below).
Google is moving toward removing support for Flash in Chrome entirely, replacing it with HTML5 as the preferred and default (and soon, the only) way to display website content. This means that unless a website has an HTML5 player, video content will not automatically display. All Flash content will be blocked, unless users manually enable Flash. (You may see a little logo in the Chrome address bar on sites that have Flash content.) In Chrome 57+, you can manage your Flash content and site-by-site exceptions in Chrome's content settings (Settings > Show advanced settings > Content settings, under "Flash").
How can you tell whether a video or game uses Flash? Right-click anywhere in the video. If the pop-up menu includes the phrase "Adobe Flash," then the video is using the Flash player.
Workaround #1: If you use Safari, turn on content blocking
Safari is the exception we mentioned above. In Safari, we can block ads in Flash videos if they are on YouTube.com. To block Flash ads anywhere else, turn on Safari Content Blocking in AdBlock's options.
Note: Are you seeing ads YouTube videos only when you're watching in full-screen mode in Safari?
This is a different issue, and it has to do with AdBlock itself. It affects both Flash and HTML5 videos. We recommend that you enable Safari Content Blocking, watch the videos in the smaller player window, or watch the videos in Chrome or Firefox.
Workaround #2: If you use Firefox, use the HTML5 Video Everywhere! and No Flash add-ons
Install the HTML5 Video Everywhere! add-on to replace a website's chosen video player with Firefox's HTML5 player. If you encounter a stubborn older website that embeds Flash content, you can install the No Flash add-on to force it to play using the HTML5 player.
There are some caveats with these add-ons. Please read their "about this add-on" information carefully.
What about Microsoft Edge and Internet Explorer?
Edge, like Chrome, Firefox, Safari, Opera, and Internet Explorer 11, uses the HTML5 video player by default. However, there is no Edge extension yet that does the equivalent of Firefox's HTML5 Video Everywhere! or No Flash add-ons to help when you encounter a Flash video. We recommend watching these videos in Chrome or Firefox.
Have you tried all these suggestions and they don't help? Are you using the Chrome browser? Does the video use the HTML5 player? Are you only seeing ads on YouTube.com? Then we have additional troubleshooting steps to try!