Similar to Popup Ads, a pop-under ad is one which appears beneath the current browser window. The host page contains JavaScript which pops up a window, and the new window's own JavaScript immediately releases the focus. This makes the window much more difficult to close, since the user must actually minimize other windows to find the offending ad. Most popunder ads contain innocuous-looking titles like "Untitled Document" to prevent users from finding them in the Taskbar or Window menu. They also appear after the user has closed all other browser windows, often confusing users as to which web site had spawned the ads.

Although X10 was the clear leader in popunder advertising during 2001, many other companies have switched to this form of advertising in lieu of banner ads or popup ads. Any program which stops pop-up ads (like Mozilla, Pop-Up Stopper, and many more) will stop pop-under ads as well. It should be noted that many spyware programs (like Gator's OfferCompanion) will spawn their own popunder advertisements when certain search terms or web sites are detected in the current browser session. This caused a minor controversy, as users accused sites like Google of using popunder ads when in fact their own computers were responsible.

Popunder ads usually provide sites with payment in the form of commissions on sales, rather than CPM (cost per thousand impressions). However, many users have come to despise the ads, and their profitability is still uncertain. It is likely that if popunder ads do not provide enough revenue, sites will switch to interstitial or superstitial ads, or subscriptions instead of advertising altogether.