Research on the Third Sector has been informed by many theories of the development of the Third Sector and its relationship to government and the citizenry. This chapter will review the major theories of the Third Sector, but pay special attention to the major comparative conceptual framework of the third sector, the social origins theory, proposed by Salamon and Anheier (1998). Overall, the chapter will place the different theoretical frameworks in context and highlight key issues for scholars of the Third Sector to consider in their future research, especially given the hybridization of the Third Sector and major shifts in public policy towards the third sector.