Regulatory changes and the rapid evolution of electronic retailing have brought a new level of uncertainty to merger review. This NERA seminar, held in San Francisco on 27 October and in East Palo Alto on 28 October, addressed merger review in the context of Internet economics and the 2010 DOJ and FTC Merger Guidelines. The Guidelines, and particularly the concept of Upward Pricing Pressure, have placed profit margins at the center of merger review. This can have consequences for mergers in high-tech industries that are characterized by high fixed costs and low marginal costs. In addition, the economics of Internet retailers and related companies complicate the traditional review of mergers: the size of the players is rapidly evolving, entry costs are low, and there is a lack of transparency about prices or even who the competitors are.