This chapter attempts to synthesize a few of the dominant theories of not-for-profit (NFP) firms into a common framework, and to connect existing empirical literature on NFPs to this common theoretical framework. It is organized as follows. Section 6.1 synthesizes existing theories of NFP behavior into a common empirical framework. The goal is to generate, for each theory, predictions regarding a common set of observable outcomes. Section 6.2 extends existing theories by generating predictions about industry-level behavior in equilibrium under perfect competition. Section 6.3 examines a large number of the existing empirical studies of NFP firms to determine the extent to which they provide evidence, comparing the behavior of NFP and FP firms on outcomes with regard to which existing theories generate different predictions. Section 6.4 concludes that, if forced to choose among existing theories, we would select theories which argue that the distinctive behavior of NFP firms can be explained by the altruistic motives of these firms' principals as most consistent with available evidence.