Abstract In this article we bring together theory and methods from two different but related fields, anthropology—specifically medical anthropology—and the cognitive sciences—specifically research on categorization and reasoning. We explore folk medical models of Mexican migrants in the greater Nashville area. The combining thread is our exploration in conceptual organization (categorization) and reasoning. We not only integrate formal methods with ethnographic research, but also integrate a set of formal tasks that together provide a better window into processes of categorization and domain organization than previously available in the literature. An interesting pattern of knowledge organization emerges integrating both real categories—as defined in the cognitive sciences—with ad hoc groups (or ad hoc categories) that show some shared features with real categories yet do not have the same conceptual status. The findings have important consequences for (1) knowledge organization as well as (2) related cultural phenomena, such as the production of knowledge through category-based induction.