This chapter summarizes insights from work on assimilation in historical political economy. To organize the findings, the chapter provides a theoretical framework in which assimilation is the result of interdependent decisions made by members of a minority group and by members of a majority group or the state that represents it. Material and psychological considerations both may affect these decisions. The framework can help explain when assimilation is more likely, when assimilation policies may succeed, and when minority backlash may ensue. Empirical evidence from various historical contexts confirms these predictions and provides additional support for the assumptions underlying the theoretical framework.