In this book, my distinguished colleagues have taken on the daunting task of summarizing the development of theory of the Third Sector over the past half century and suggesting how this body of theory can usefully be extended and modified to address the changing character of the contemporary third sector. In this chapter, I critique their work, by pointing out misconceptions about early economic theory of the sector, noting that this theory also helps explain the behavior as well as existence of nonprofit organizations, highlighting where early economic theory of the sector continues to be relevant, questioning the efficacy of social origins theory, and arguing that an expanded theory should encompass the phenomenon of social enterprise.