Adequacy is a derived concept whose origins in the school finance literature probably stem from San Antonio Independent School District v. Rodriguez (1973), a landmark federal case. Although there is often a recycling of definitions, 3 decades later, there is still little consensus on how to operationalize that concept. The purpose of this article is to synthesize the different conceptualizations of adequacy and to document its evolution in the field. A critical historical mapping lays the groundwork for highlighting the future direction of research. This article relies on economical, sociological, and legally based perspectives, framing the discussion of adequacy around the notions of (a) adequacy of educational inputs, (b) adequacy in school processes, and (c) adequacy of educational outputs. Each focus draws on school-centered explanations of differences in attainment. Consequently, adequacy research looks largely at how policymakers can appropriately divide resources among schools (and ultimately students) to achieve desired results. There is a long-established focus in the literature on the appropriate levels of inputs and outputs in discussions of the adequacy of the education production function. Largely missing from that discussion, however, is a more detailed examination of the adequacy of the process, which speaks to the capacity of institutions to meet various standards set.