There are several reasons you could be seeing ads in YouTube videos. Here are some of them, along with the workarounds we know about.
Please note, however, that we're big fans of supporting YouTubers you love by allowing them to show you ads. You can easily whitelist specific YouTube channels in Chrome, Edge, Firefox, and Opera. (Safari doesn't allow us to offer this feature yet.)
You're affected by the "universal whitelist" bug in AdBlock for Chrome 3.17
Open the CUSTOMIZE tab in AdBlock's options and look at the custom filters box. If you see "@@" on a line by itself, please see this article.
This bug was fixed in AdBlock for Chrome 3.18.0.
Your computer needs a reboot
When something on your computer isn't working right, try rebooting your computer. Rebooting solves 95% of all known computer problems. Seriously.
You're using the YouTube app
AdBlock is a browser extension. We can only block ads that appear on web pages you visit in your browser. If you use a mobile browser that supports AdBlock (currently, that's Safari on iOS devices and Samsung Internet on certain Android devices), then try watching videos on Youtube.com. If you use a different browser or you prefer the YouTube app, there are alternate ways to block ads on a mobile device.
Try a fresh installation of AdBlock
Uninstalling and reinstalling AdBlock and restarting your browser often makes ads go away on any site where they're not being blocked properly. (If you have any custom filters, be sure to back them up in a text file before you uninstall AdBlock! You'll need to add them back in after you reinstall it.)
Note: If reinstalling AdBlock doesn't help, try reinstalling Chrome (see below). Then uninstall and reinstall AdBlock. This will make sure that you have a fresh, uncorrupted copy of both Chrome and AdBlock.
You told AdBlock not to block ads on YouTube
Your filter lists are out of date
Sometimes all you need is to update the filter lists you're subscribed to. They're updated automatically on a schedule set by the list maintainers, but you can force an update at any time. Here's how to manually update your filter lists on desktop and mobile AdBlock.
You're using Firefox, Microsoft Edge, or Safari (without content blocking enabled)
It could be the video player's fault. Although the newer HTML5 player is now the default in most modern browsers, you can still come across older Flash videos. AdBlock can't block ads in Flash videos, except in Safari with content blocking enabled. All is not lost, however. We have some workarounds.
The Chrome YouTube bug is preventing ads from being blocked
This was particularly prevalent late in 2015, but it still crops up occasionally. The bug affects not just YouTube but Pandora and Spotify as well. If you're using Chrome and you suddenly see ads in YouTube videos after a Chrome update, try removing the YouTube app.
You're watching YouTube videos in full-screen mode in Safari
There's a known issue in AdBlock for Safari (and Safari only) where ads are not blocked on YouTube when the video player is in full-screen mode. We recommend that you enable Safari Content Blocking in AdBlock's options (or watch videos in the smaller player).
Another workaround is to exit full-screen mode before starting another video.
You can also try watching full-screen YouTube videos in Chrome or Firefox, even if you keep Safari as your browser for everything else.
Your browsing data may need to be cleared or your browser settings may need to be reset
You're logged in to YouTube
Some users have told us that they only see ads when they are logged in to their YouTube account. Try logging out, or watching videos in an incognito or private window.
Another extension is interfering with AdBlock
Disable all of your extensions except for AdBlock. Try to watch your video again. If that helps, one of the extensions you disabled is somehow interfering with AdBlock. For instance, we've heard that Magic Actions for YouTube can interfere with ad blockers. If you need to use one of these extensions, you might want to notify the extension's developers so they can fix the issue that's causing the interference.
The video is embedded in a website that uses the Adobe Flash player
HTML5 is now the default player for videos on YouTube.com in Chrome (as well as most other popular browsers). We can block ads just fine in the HTML5 player. It isn't possible to block ads in Adobe Flash video, however. Some websites may still be using the Adobe Flash player for video content, so embedded YouTube videos on those sites may have unblocked ads. We have more information and some workarounds in our Knowledge Base.
On a related note, Google is moving toward removing support for Flash in Chrome entirely, replacing it with HTML5 as the preferred and default way to display website content. This means that unless a website has an HTML5 content 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").
Chrome may be corrupted
Warning: Uninstalling Chrome this way will completely erase your bookmarks, saved passwords, extensions, and Chrome settings on your computer. Before you proceed, be sure to turn on Google Chrome sync and synchronize everything through your Google account!
Google is trying something new
Google, like all content providers that rely on ad revenue, constantly tries to come up with ways to circumvent ad blocking. The filter list maintainers usually come up with a new filter to get around it, which goes out with the next list update. That's why updating your filter lists is generally all it takes to remove ads.
I've tried everything you suggested and I'm still seeing ads!
If none of these seems to cover your particular issue, feel free to open a ticket on our help site and we'll be happy to take a look.
Bonus tip #1: Annotations aren't ads
Technically, YouTube annotations aren't ads. They are kind of like advertising, though, and like ads they can be intrusive and annoying. To stop seeing them, this article from HowToGeek has instructions on how to disable annotations on YouTube videos
Bonus tip #2: Try an old experimental feature
If all else fails, you can try this suggestion from GHacks.net by way of LifeHacker. The article is from 2013, so the experiment it discusses may no longer be running. But hey, nothing ventured, nothing gained! (If you try it and it works, please let us know in our Tips & Tricks forum.)
Bonus tip #3: Try refreshing the page
We got this tip from Twitter. If you encounter one of those long unskippable ads and the above suggestions don't help, press F5 to reload the page. We have no idea whether this works; as stated above, we never see the ads ourselves so we can't test. It certainly won't hurt to try!