Marriage is on the decline. Men and women of the youngest generation are either marrying in their late twenties or not marrying at all. Childbearing has also been postponed, but not as much as marriage. The result is that a growing proportion of children are born to unmarried parents--roughly 40 percent in recent years, and over 50 percent for children born to women under 30. Many unmarried parents are cohabiting when their child is born. Indeed, almost all of the increase in nonmarital childbearing during the past two decades has occurred to cohabiting rather than single mothers. But cohabiting unions are very unstable, leading us to use the term "fragile families" to describe them. About half of couples who are cohabiting at their child's birth will split by the time the child is five. Many of these young parents will go on to form new relationships and to have additional children with new partners. The consequences of this instability for children are not good. Research increasingly shows that family instability undermines parents' investments in their children, affecting the children's cognitive and social-emotional development in ways that constrain their life chances. Whereas most scholars now agree that children raised by two biological parents in a stable marriage do better than children in other family forms across a wide range of outcomes, there is less consensus about why. Is it the quality of parenting? Is it the availability of additional resources (time and money)? Or is it just that married parents have different attributes than those who aren't married? Thus a major theme addressed in this issue is "why marriage matters" for child wellbeing.