Cohabitation is a family form that increasingly includes children. We use the National Longitudinal Study of Adolescent Health to assess the well-being of adolescents in cohabiting parent stepfamilies (N= 13,231). Teens living with cohabiting stepparents often fare worse than teens living with two biological married parents. Adolescents living in cohabiting stepfamilies experience greater disadvantage than teens living in married stepfamilies. Most of these differences, however, are explained by socioeconomic circumstances. Teenagers living with single unmarried mothers are similar to teens living with cohabiting stepparents; exceptions include greater delinquency and lower grade point averages experienced by teens living with cohabiting stepparents. Yet mother's marital history explains these differences. Our results contribute to our understanding of cohabitation and debates about the importance of marriage for children.