Major paradigms in immigration research, including assimilation theory, multi-culturalism, and ethnic studies, take it for granted that dividing society into ethnic groups is analytically and empirically meaningful because each of these groups is characterized by a specific culture, dense networks of solidarity, and shared identity. Three major revisions of this perspective have been proposed in the comparative ethnicity literature over the past decades, leading to a renewed concern with the emergence and transformation of ethnic boundaries. In immigration research, “assimilation” and “integration” have been reconceived as potentially reversible, power-driven processes of boundary shifting. After a synthetic summary of the major theoretical propositions of this emerging paradigm, I offer suggestions on how to bring it to fruition in future empirical research. First, major mechanisms and factors influencing the dynamics of ethnic boundary-making are specified, emphasizing the need to disentangle them from other dynamics unrelated to ethnicity. I then discuss a series of promising research designs, most based on nonethnic units of observation and analysis, that allow for a better understanding of these mechanisms and factors.