This introductory chapter begins with discussion of the emergence of the pentecostal movement. It then describes the Assemblies of God (AG), arguably the best-known of the Pentecostal denominations. The remainder of the chapter explains the research methodology used in this study and then sets out the book's purpose, i.e. to present a broad narrative, complete with statistical findings, of a prototype of Pentecostalism and the issues facing this American-born and -bred approach to Christianity that has become a global force. At its core the AG is a historic Pentecostal denomination, but one that has been influenced by subsequent pentecostal renewals and revivals as well as by the countervailing force of evangelical Protestantism. The book focuses on the effects of these two seemingly countervailing forces on the AG in the United States.