This book provides a brief history of peer-to-peer technology in the new millennium, before discussing what it terms the "peer-to-peer spectrum," which expands the idea of what a peer-to-peer network is, beginning with the pure, user-to-user system, and ending with those systems that can include content providers, and Cloud services. The book also clarifies the often-misunderstood notion of a "peer-to-peer overlay network," and notes that this can be implemented either regionally or globally. In addition, given the necessary, in-depth, technical description of the networking preliminaries, security, and protocols required to write a mediated, peer-to-peer system, the book then proceeds to describe in detail such networks and protocols required to implement a complete, mediated peer-to-peer system.