Cohabitation, or living together in an intimate sexual relationship outside of marriage, has become normative across many world regions. This chapter reviews and evaluates knowledge on heterosexual cohabitation, with an emphasis on recent research in the United States. In this chapter, we first review basic trends and patterns regarding cohabitation in the United States, followed by a discussion of cohabitation patterns around the globe. We next identify four major research questions central to understanding recent research on cohabitation and synthesize the findings. Finally, we discuss several implications of the current state of knowledge about cohabitation.