This article uses the history of the National Tax Association (NTA), the leading twentieth-century organization of tax professionals, to strengthen our empirical understanding of the disciplinary encounter between law and the social sciences. Building on existing sociolegal scholarship, this article explores how the NTA embodied tax law's ambivalent historical interaction with public economics. Since its founding in 1907, the NTA has changed dramatically from an eclectic and catholic organization of tax professionals with a high public profile to an insular, scholarly association of mainly academic public finance economists. Using a mix of quantitative and qualitative historical evidence, we contend that the transformation in the NTA's mission and output can be explained by the increasing professionalization and specialization of tax knowledge, and by the dominant role that public economics has played in shaping that knowledge. This increasing specialization allowed the NTA to secure its position as a bastion of scholarly tax research. But that achievement came at a cost to the organization's broader civic mission. This article is thus a historical account of how two competing professional disciplines—tax law and public economics—have interacted within a particular organizational field, namely the research and analysis of tax law and policy.