Prior research has established that children from traditional, two-parent nuclear families experience a lower risk of delinquency than children raised in alternative family structures. However, many studies have ignored the effect of parental cohabiting on delinquent development. A growing body of research suggests that cohabiting (even among biological parents) may be harmful to children. This study tests the hypothesis that cohabiting is associated with four different types of delinquent behavior. It examines two theoretical models, a family stress model and a community stress/selection model, as possible explanations of ‘‘the cohabiting effect.’’ The analysis reveals that cohabiting is generally associated with increased risk of misbehavior (although the effects do vary somewhat by type of delinquency). Although the theoretical models could not completely explain ‘‘the cohabiting effect,’’ substantial evidence of both mediation and moderation is found. The implications of the findings are discussed.