Abstract Using data from Waves 1 and 2 of the National Longitudinal Study of Adolescent Health, I extend prior research on family transitions and adolescent well-being by examining the influence of parental marital and cohabitation transitions on adolescent delinquency, depression, and school engagement. Adolescents who experienced a family transition reported decreased well-being, on average, relative to those in stable, two-biological-parent families. Specific comparisons of various types of family stability and change revealed that cohabitation is often associated with poorer outcomes. Moving out of a cohabiting stepfamily into a single-mother family was not harmful and was actually associated with improvements in school engagement. Moving into a cohabiting stepfamily from a single-mother family decreased adolescent well-being, and this impact was greater than that experienced by those who moved into a married stepfamily. Stable cohabiting stepfamilies were associated with lower levels of well-being than stable married stepfamilies. Formalization of a cohabiting stepfamily through marriage did not translate into any appreciable benefits for adolescent well-being.