This report is Part I of the larger project on Internet intermediaries. It develops a common definition and understanding of what Internet intermediaries are, of their economic function and economic models, of recent market developments, and discusses the economic and social uses that these actors satisfy. The overall goal of the horizontal report of the Committee for Information, Computer and Communications Policy (ICCP) is to obtain a comprehensive view of Internet intermediaries, their economic and social function, development and prospects, benefits and costs, and responsibilities. It corresponds to the item on 'Forging Partnerships for Advancing Policy Objectives for the Internet.