The definitive presence of a huge Muslim population in Europe will have long-term consequences. There is, nevertheless, some debate about the size of the Muslim population, partly due to imprecise data, partly due to the difficulty of knowing who qualifies as Muslim. Is one defined as a Muslim because of one’s choice of religious community, or is one a Muslim by ethnic background? Beyond the demographic aspect, the fact that Islam is taking hold in Europe seems to call into question European identity. What does the rise of Islam in Europe entail in terms of shared culture and values?