Research Findings: In this examination of associations between sibling interaction patterns and later social outcomes in single- and 2-parent families, 113 kindergarteners took part in naturalistic observations at home with siblings, classmates participated in sociometric interviews, and teachers completed behavior ratings. Sibling interactions were coded using a newly developed 39-item checklist, and proportions of complementary and reciprocal sibling interactions were computed. Complementarity occurred more among dyads of kindergartners with toddler or infant siblings than among kindergartners with older or near-age younger siblings. Higher levels of complementarity predicted lower levels of internalizing problems but were not related to externalizing problems. Kindergartners' sociometric status in the classroom differed as a function of sibling interaction patterns, with neglected and controversial children experiencing less complementarity/more reciprocity than popular, average, and rejected children. Finally, there was some evidence for differential associations of sibling interaction patterns with social outcomes for children in single- versus 2-parent families: Regressions testing interaction effects showed that sibling reciprocity was positively associated with kindergartners' social skills only in single-parent families, and complementary sibling interactions were positively related to internalizing problems only in 2-parent families. Practice or Policy: Those working with divorcing or other single-parent families might consider sibling interactions as a potential target for social skill building.