Religious affiliation as a determinant of demographic behavior is receiving renewed attention in demography. Interest in the role of cultural factors in affecting fertility and a specific concern with the role of Islam in many developing countries have helped re-invigorate research on the role of religion. This article reviews theoretical and empirical work on that relationship, with special attention to a number of cases in which religion has been identified as an important determinant of fertility patterns. The article concludes that religion plays an influential role when three conditions are satisfied: first, the religion articulates behavioral norms with a bearing on fertility behavior; second, the religion holds the means to communicate these values and promote compliance; and, third, religion forms a central component of the social identity of its followers.