Interventions to promote healthy marriages and strengthen families with unmarried parents are most likely to succeed if they are based on a sound understanding of the determinants of union formation, stability, and quality. With this in mind, the Administration for Children and Families contracted with Abt Associates for a review of the quantitative research on this subject. The review was to focus on the influences most relevant to policy. Its objective was to point to important gaps in knowledge and data and analyses needed to fill them. Evidence on disadvantaged families was to receive special emphasis given the compelling societal interest in decreasing the number of poor children growing up in single-parent families. The project also was to provide a companion guide to major national surveys offering opportunities to address key unanswered questions.