In this paper the authors report on cohabitation and marriage data coming from the third wave of the National Longitudinal Study of Adolescent Health, or Add Health, a national longitudinal study of adolescents and young adults beginning in 1995 that has been funded by NICHD and 17 other federal agencies. Add Health respondents were aged 18-26 in Wave III (2001-02) when romantic relationships are particularly salient in young people's lives and tend to become more serious and intimate as they take on adult roles and responsibilities. Add Health employed several innovative methods to measure cohabitation and to better understand the relationship dynamics of cohabiting unions in ways that are similar to marital unions. They present two different ways to measure cohabiting unions according to the length of time a couple has "lived together" and the implications of different definitions for the levels of cohabitation in the Add Health sample. They also develop measures that capture domains of relationship functioning, quality, and intimacy and contrast these aspects of relationships by cohabitation and marital status and by the duration of the relationship. Finally, they are able to contrast cohabiting relationships with married relationships according to whether cohabitation preceded marriage to obtain further insights into the different contexts of these relationships and the extent to which we can observe a continuum of relationship intensity in our various measures across the different types of relationships.