For years, mobile publishers sold each impression through a Waterfall: demand sources were ranked in a fixed order and offered the impression one after another until someone took it. In-app bidding replaces that queue with a single auction where every source bids at once. It is sometimes called header bidding for apps, and it exists to solve a simple problem, that the highest bidder should win each impression rather than the source that happened to sit near the top of the list.

How it works

When a placement is about to show an ad, the bidding system asks all connected demand sources for a price in real time. Each source returns a bid, the auction compares them, and the highest valid bid wins and serves. This runs on the same Real-Time Bidding (RTB) principles used across programmatic advertising, applied inside the app through a mediation or bidding layer. Because every source is priced on the same impression at the same moment, the publisher sees the true market value of each slot instead of an estimate.

In-app bidding vs. waterfall

In a Waterfall, sources are ranked by their historical average price, and each gets a chance in turn. A source with a strong bid on one particular impression may never get asked, because a higher-ranked source with a weaker average took it first. The order also needs constant manual tuning. In-app bidding removes the ranking. Every source competes on every impression, which is fairer, tends to lift prices, and cuts the manual work of maintaining tiers. The main cost is a more complex integration and a heavier reliance on the Ad Mediation layer to run the auction.

Why publishers adopt it

The headline reason is yield. When sources compete directly, the price of each impression rises toward what the market will actually pay, which shows up as higher eCPM. Bidding also reduces guesswork, since publishers spend less time reordering tiers and more time on strategy. And it scales better, because adding a new demand source means adding another bidder to the auction rather than finding the right rung on a ladder.

Where offerwalls fit

An offerwall usually sits beside the bidding auction rather than inside it. It monetizes on completed offers instead of individual impressions, so it works as a separate rewarded revenue stream that a publisher offers users directly. In practice, in-app bidding maximizes the value of standard ad impressions while the offerwall captures value from users who prefer to earn a reward by completing an action. The two are complementary, and many publishers run both to cover different parts of their audience.

Common misconceptions

• That bidding replaces mediation. Bidding runs inside a mediation layer, which still handles integration, reporting, and any remaining waterfall lines.

• That every source must be a bidder. Many stacks are hybrid, with some sources bidding and others still called in a waterfall while they transition.

• That higher bids always mean more total revenue. Yield also depends on fill and user experience, so a higher price on fewer served impressions can net less.