This document discusses the evolution of digital advertising and programmatic buying. It describes how the market has shifted from advertisers and agencies working directly with publishers, to a more complex ecosystem involving networks, trading desks, data providers and other players. Real-time bidding allows automated buying of digital ads at large volumes and scales. Programmatic buying uses real-time bidding to make automated transactions of digital ads across an overcapacity of inventory in a second price auction system. Data and new metrics around conversion attribution are also changing the business models in this space.