Major paradigms in immigration research, including assimilation theory, multiculturalism, 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, powerdriven 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.This article aims to advance the conversation between students of comparative ethnicity and scholars of immigration. 1 This conversation has given rise to a new concern with ethnic boundary-making in immigrant societies. Instead of treating ethnicity as an unproblematic explanans-providing self-evident units of analysis and self-explanatory variables-the boundary-making paradigm takes ethnicity as an explanandum, as a variable outcome of specific processes to be analytically uncovered and empirically specified. The ethnic boundary-making perspective has particular advantages for the study of immigrant societies, as a number of authors have suggested recently. for inviting me to the above venues. My departmental colleagues Rogers Brubaker, Adrian Favell, and Roger Waldinger offered generous advice and criticism that I wish I had been able to take more fully into account. Wes Hiers was kind enough to carefully edit the final version (and to teach me that "that" and "which" are not the same). 1 The argument offered here draws on Wimmer (1996, from which the title is adapted) and Wimmer and Glick Schiller (2002). For other critiques regarding the (ab)use of the concept of ethnicity, see Bowen (1996) and Brubaker (2004:Ch. 1) for conflict research, Brubaker (2004:Ch. 2) for studies on collective identity, and Steinberg (1981) for immigration studies.