Cloaking is a search engine optimization technique in which the content presented to the search engine spider is different from that presented to the users' browser; this is done by delivering content based on the IP addresses or the User-Agent HTTP header of whatever is requesting the page. The only legitimate uses for cloaking used to be for delivering content to users that search engines couldn't parse, like Macromedia Flash. However, cloaking is often used to try to trick search engines into giving the relevant site a higher ranking; it can also be used to trick search engine users into visiting a site based on the search engine description which site turns out to have substantially different - or even pornographic - content. For this reason some search engines threaten to ban sites using cloaking.

Cloaking is a form of the doorway page technique.

A similar technique is also used on the Open Directory Project web directory. It differs in several ways from search engine cloaking:

• It is intended to fool human editors, rather than computer search engine spiders.
• The decision to cloak or not is based upon the HTTP referrer, which tells the URL of the page on which a user clicked a link to get to the page. Some cloakers will give the fake page to anyone who comes from a web directory website, since directory editors will usually examine sites by clicking on links that appear on a directory webpage. Other cloakers give the fake page to everyone except those coming from a major search engine; this makes it harder to detect cloaking, while not costing them many visitors, since most people find websites by using a search engine.

In more recent times several well known and well respected sites have taken up cloaking to deliver personalised content to their regular customers. In fact, many of the top 1000 sites - including household names like Amazon.com - actively cloak. None of these have been banned from search engines purely because of cloaking.

Increasingly, to rank well in the search engines, Webmasters must design pages solely for the search engines. This results in pages with too many keywords and other factors that might be search engine "friendly", but make the pages difficult for actual endusers to consume. As such, cloaking is an important technique to allow Webmasters to split their efforts and separately target the search engine spiders and endusers. As with anything, this technique can be used responsibly, or less so.