This document provides a history and overview of key trends in the digital advertising industry from the 1990s to present. It discusses the fragmentation of media sources and audiences, the growth of automation and programmatic buying through real-time bidding exchanges. The complexity of the current ecosystem is examined, with a prediction that consolidation among the hundreds of vendors is needed. The roles of various players like advertisers, agencies, exchanges, and publishers are mapped out within the programmatic advertising model.