Three trends since the 1960s underscore the need for different ways of conceptualizing the new mixed economy in the human services. First, there has been an enormous increase in the number and types of nonprofit organizations, and greater dependence on governmental revenue. Second, extensive growth in privatization and commercialization in the human services. Third, this culminated in the convergence and blurring of sectoral boundaries. Numerous metaphors have been suggested to describe these new patterns, but more suitable concepts and theories are needed. Four theoretical frameworks are analyzed for an intersectoral study of organizations in the same industry: (1) political economy, (2) organizational ecology, (3) neoinstitutionalism, and (4) mixed, open systems. As analytic paradigms, these frameworks could supplement, complement, or be integrated with other research models for third sector studies, and could contribute to theory building and social policy.