An Ad Exchange is a digital advertising marketplace where ad space is bought and sold automatically, in real time (RTB). It connects advertiser platforms (DSPs) with publisher platforms (SSPs) and runs auctions for individual impressions in a fraction of a second. It is one of the key elements of the programmatic ecosystem: it decides whose ad shows to a given user, and at what price. The largest include Google AdX, Magnite, OpenX, PubMatic and Index Exchange. Worth knowing: although Poland has produced significant ad technologies, such as AdOcean and AdTaily (today Yieldbird), the Ad Exchange market is dominated by global platforms, led by Google Ad Exchange. (Compare: Programmatic Advertising, RTB, DSP, SSP)