The article scrutinizes the development of the concept of ethnicity in the 20th century in connection with conceptualizations of the contemporary process of migration. The concept of ethnicity is widely employed in political debates about the status of immigrants in the Federal Republic of Germany and exercises a profound influence on sociological studies of migration. The author differentiates between two dominant perspectives on ethnicity in sociological studies. The primordial approach views ethnicity as a given cultural entity. The situational (interactive) perspective suggests that ethnicity is a tool, which is used by people in different situations and to different ends. The study traces the reception of the concept of ethnicity in sociological studies from the development of the US sociology of immigrant communities. The author observes that the dominant trend in German sociological studies of immigrants is informed by the primordialist concept of ethnicity, which interprets the specificity of immigrants’ social life through the essentially understood concept of culture. The author reviews features of the situational approach to the concept of ethnicity and suggests that ethnicity of immigrants emerges as a result of treatment in the host country, which includes the perception of inborn population and the state’s policy toward the immigrants. Establishing a link between the reception of immigrants in host-countries and ethnicization of immigrant communities, the author maintains that whereas multiculturalism and the dominant primordialist perspective on ethnicity could only foster the ethnicization and isolation of immigrants, the constructivist approach could reveal complex and open-ended ways of immigrants’ accommodation to the life in host countries.