There has been a sharp increase over the last two generations in the proportion of American children who do not live with their own two married parents, first spurred largely by increases in divorce, and more recently by large jumps in unmarried or cohabitating childbearing. A vigorous public debate sparked by these changes in family structure has generated a growing body of social science literature on the consequences of family fragmentation. This report is an attempt to summarize this large body of scientific research into a succinct form useful to Americans on all sides of ongoing family debates — to report what we know about the importance of marriage in our family and social system. (Author abstract)