This annotated brief summarizes research on the effects of family structure on child wellbeing, identifying issues that remain to be explored. On average, children who grow up in families with both their biological parents in a low-conflict marriage are better off in several ways than children who grow up in single-, step-, or cohabiting-parent households. Compared to children raised by their married parents, children in other types of families are more likely to achieve lower educational levels, become teen parents, and experience health, behavior, and mental health problems. Also, children in single- and cohabiting-parent families are more likely to be poor. Nonetheless, most children not living with married, biological parents grow up without serious problems. In individual situations, marriage may or may not make children better off, depending on whether the marriage is "healthy" and stable. Marriage may be a proxy for other parental characteristics associated with relationship stability and positive child outcomes. The legal basis and public support involved in the institution of marriage helps create the best conditions for developing factors that children need most to thrive: consistent, stable, loving attention from two parents who cooperate and who have sufficient resources and support from two extended families, two sets of friends, and society. (Contains 52 endnotes.) (SM)