An ad exchange is a technology platform that facilitates the buying and selling of media advertising inventory from multiple ad networks. Prices for the inventory are determined through bidding. The approach is technology-driven as opposed to the historical approach of negotiating price on media inventory. This represents a field beyond ad networks as defined by the Interactive Advertising Bureau (IAB), and by advertising trade publications such as Advertising Age.
The major ad exchanges include:
- DoubleClick,  was acquired by Google in 2008. DoubleClick Ad Exchange connects ad networks, agencies and third-party demand-side platforms with a vast global inventory in real time.
- AdECN, which is owned and was purchased by Microsoft in August, 2007 (Microsoft officially switched from AdECN to AppNexus for its real-time bidding needs — three years after it acquired it). This change finally retired the AdECN platform.
- Rubicon Project Exchange, which is owned by Rubicon Project.
- Open X OpenX is a real time advertising technology company. The company has developed an integrated technology platform that combines ad server and a real-time bidding exchange with yield optimization for advertising and digital media companies Open X.