What is an ad exchange?
An ad exchange is a digital marketplace where ad impressions are bought and sold in automated auctions. Publishers, usually through SSPs, make impressions available; advertisers, usually through DSPs, bid on them. The exchange runs the auction, picks the winner and handles billing. Google AdX is the largest, and most modern SSPs also operate exchanges.
Early on, an ad exchange was distinct from an ad network (which bundled inventory and resold it) and from an SSP (the publisher's selling tool). Today the terms overlap: Magnite, PubMatic, Index Exchange and others are SSP-exchanges. The exchange's job is neutral matching via real-time bidding and the OpenRTB protocol.
Neutrality is the heart of the US antitrust case: the court found in 2025 that Google unlawfully monopolized the ad exchange market with AdX and tied it to its publisher ad server. Exchanges earn a take rate on each transaction.