Cohabitation is a rapidly changing aspect of family life in the United States and Britain. This article describes the demography of cohabitation, considers the place of cohabitation in the kinship system, and speculates on the future of cohabitation. I argue that three processes—cohort replacement, socialization that occurs when children live with cohabiting parents, and social diffusion—will foster continued increases in rates of cohabitation. These processes are also likely to increase variation in the types of cohabiting relationships that couples form. Understanding the meaning of cohabitation in the kinship system requires distinguishing between individuals' attitudes about their own relationships and the composition of cohabiting unions at the population level.