This federally funded brief discusses how the rise in the popularity of cohabiting is linked to other important shifts in family patterns in the United States, and provides information on the number of cohabiting couples with children, different types of cohabitation, and research findings on cohabitation. Evidence is cited that indicates cohabiting before marriage is a risk factor for divorce and that married couples who lived together before marriage both report and are observed to have worse communication, less marital satisfaction, and more physical violence than couples who did not live together before marriage. The effects of cohabitation on children are also discussed, as well as legal issues related to cohabitation. 27 references.