Programmatic · also called exchange, ADX
Ad exchange
An ad exchange is a digital marketplace where buyers and sellers of ad impressions meet and trade through automated auctions, usually in real time.
An ad exchange is the trading floor of programmatic advertising. Sellers (publishers, often via SSPs) put impressions up for sale; buyers (usually via DSPs) bid on them; the exchange runs the auction, picks the winner and settles the money.
Exchanges emerged in the late 2000s as an alternative to ad networks, which bundled and marked up inventory. Right Media, DoubleClick Ad Exchange (now Google AdX) and AppNexus were early examples. Today most SSPs operate their own exchange, so the distinction is mainly historical.
Google's AdX is at the centre of the United States v. Google case. In April 2025 a US federal court found that Google had unlawfully monopolised the publisher ad server and ad exchange markets and unlawfully tied the two together; a remedies trial followed in September 2025. Exchange rules, such as auction type, floors and last look privileges, shape how much publishers earn and what buyers pay.
Think of it like this
An ad exchange is like a stock exchange for ad space: buyers and sellers never meet in person, but the exchange matches their orders in milliseconds.
An example
On one exchange, a 300×250 impression from a UK finance site receives bids from 9 DSPs; the exchange clears it at £4.20 CPM in a first-price auction.
Related terms
SSP (supply-side platform)
An SSP (supply-side platform) is software that publishers use to sell their ad inventory automatically to many buyers, running auctions and managing prices, deals and quality controls.
DSP (demand-side platform)
A DSP (demand-side platform) is software that advertisers and agencies use to buy ad impressions automatically across many exchanges and publishers, deciding which impressions to bid on and how much.
Real-time bidding (RTB)
Real-time bidding (RTB) is an automated auction in which advertisers bid on a single ad impression in the fraction of a second while a web page, app or stream is loading.
Ad network
An ad network is a company that aggregates ad space from many publishers and resells it to advertisers, usually in packaged bundles and often with its own pricing and targeting.
United States v. Google (ad tech)
United States v. Google (ad tech) is the US Justice Department's antitrust case in which a federal court ruled in April 2025 that Google illegally monopolised key open-web ad tech markets.