A pop-under opens a full browser tab behind the active window. The user sees it when they close or minimise what they were doing.
Specs
Full window; creative is a URL redirect rather than an image. Frequency capping and geo/device targeting apply. Desktop and mobile web. Pricing: CPM.
Best for: performance, sweepstakes and app installs.
Are they effective?
For direct-response, often yes — a full-window placement at low CPM delivers volume that display can't match on cost. It is a performance format, not a branding one, and should be judged purely on cost per outcome.
Where it goes wrong
- Landing page quality decides everything. The user didn't choose to be there, so the page has to earn attention in seconds. Use a The landing-page guard backup URL — a dead page on pop traffic wastes budget faster than on any other format.
- Frequency capping is essential. Uncapped pop is genuinely hostile and burns the traffic source.
- Watch traffic quality closely. Pop inventory attracts more invalid traffic than most formats. Real-time fraud protection and blacklists matter more here — see Is fraud detection and prevention included?.
A note on fit: pop suits some verticals and offends others. Check that it matches your brand position before scaling it.
Related: What makes push notification advertising effective?