The dissertation investigates the role of religion and religious difference in the process of assimilation between immigrants and native populations in Western Europe. To do so, it asks a set of interrelated research questions: how does cultural difference affect assimilation? How do Muslims immigrant population culturally adapt to the secular context of Western European nation-states? How does upward mobility impact the acculturation patterns and experiences of religious stigma among the rising immigrant elite? Three separate studies provide answers to these questions. The first study is a large, theoretically-driven review of the last decade of immigrant incorporation scholarship in America and Western Europe. Through a comparative lens, it identifies large empirical trends toward assimilation, but also the unique role played by religious and cultural difference in the European context - a role not foreseen in assimilation theory. The second study uses large-scale survey data from France to investigate assimilation between Muslim immigrants and natives in terms of religiosity. Using a unique research design, it uncovers the role of parental socialization and perceived discrimination in shaping a religiosity surplus among Muslims compared to the reference population. The third article, a qualitative analyses of the subjective experience of upwardly mobile immigrants in France, uses thirty-eight in-depth interviews to provide a first empirical look at the rising immigrant elite. It shows that non-Muslim immigrants typically feel they have achieved status and respect in the French community, while Muslim immigrants generally still feel like cultural outsiders despite high levels of socioeconomic attainment.