This chapter shows that Mexican indigenistas eschewed grand theories and approached modernization within a framework that was local and empirical. As a Mexican socialism took hold in the 1930s, they increasingly placed economics and class—rather than ethnicity, culture, or race—at the center of national policies, and they accepted forms of cultural difference that they viewed as compatible with economic progress. Their approach to economics, however, was ethnographic, focusing on specific localities and insistently documenting experts’ inability to reach broader conclusions about the characteristics of the peoples they studied. Somewhat paradoxically, their approach was fundamentally evolutionary, and this chapter examines the complex ways indigenistas reconciled their belief in progress and science with attention to particularity, including strategies of compilation, categorization and taxonomy, and statistical aggregation.