Please see our blog for more about how ad blocking works and how you can make the most of AdBlock. If you're new to AdBlock, check out our user guide: Getting started with AdBlock.
AdBlock, like all ad blockers, relies on filter rules to know what to block, hide, and (in the case of allowlisted sites) allow to appear on the web pages you visit. AdBlock compares every HTTP (web page) request to the filter lists you're subscribed to and any custom filters you have added. If the URL (web address) of the request matches one of the filters, the request is blocked and the resource is not downloaded.
A common way of displaying ads is to encode an image using data:image/png;. This method doesn't trigger a normal HTTP request. That means ads called in this way can't be blocked. Instead, AdBlock injects a stylesheet onto every web page. The stylesheet contains selectors that are set to display:none !important, which hides the ads on the page.
Where do the blocking and hiding filters come from?
Filter lists
- Most of the filter rules come from EasyList, which is a huge collection of known ads on English-language websites all over the Web. EasyList is the primary ad-blocking filter list that all ad blockers use.
- You can also subscribe to additional filter lists that block things other than ads, such as certain annoying aspects of Facebook, social media buttons and in-page pop-ups, and ads on sites in languages other than English.
- Additionally, there's the Acceptable Ads filter list, which supports websites by allowing some non-intrusive advertising to appear. Acceptable Ads is enabled by default when you install AdBlock.
- Finally, you can subscribe to still other filter lists if you know their URL (web address).
Custom filters
These are rules you provide to AdBlock that aren't in a filter list.
- You can add them manually, by writing them yourself following established filter rule syntax.
- You can use our wizard to create custom filters without knowing how to write them yourself.
- You can also import filters from other ad blockers and from posts in the EasyList forums. (Just visit EasyList's Report Unblocked Content forum and search for the name of the site where you're seeing an ad.)
For more information, please see: Introduction to Filter Lists.