A considerable amount of research has established that the married live longer, healthier lives than the previously- and never-married. Similar research on the health benefits of cohabitation is sparse, and virtually nonexistent from adolescence into young adulthood despite substantial levels of cohabitation at these ages. Using longitudinal data from Add Health (1995-2001/2002) and generalized linear model techniques the authors investigate the impact of nonmarital cohabitation and marriage on a range of physical and mental health indicators and health behaviors. They also consider the mechanisms through which cohabitation affects health (i.e., selection and protection) and contrast the health effects of cohabitation with those reported for marriage at these relatively early ages. Results indicate that the health benefits of marriage among this sample are weaker than expected based on previous studies of marriage and health, but broader than those for cohabitation. This is not unexpected given the relatively young ages of marriage in Add Health compared to other datasets containing respondents at older ages, which comprise much of the previous marriage and health literature. Cohabitors report lower physical health than married or single individuals, but that cohabiting males receive some mental health benefits relative to singles. Cohabiting men and women also engage in some better health behaviors than singles. There also appears to be some selection into cohabitation and marriage by individuals with relatively poor mental health and health behaviors that may contribute to the observed health differentials.