In this chapter we present the perspective that social groups serve as moral boundaries. Social groups establish the bounds within which people hold moral obligations toward one another. The belief that people are morally obligated toward fellow social group members, but not toward members of other groups, is an early-emerging feature of human cognition, arising out of domain-general processes in conceptual development. We review evidence that supports this account from the adult and child moral cognition literature, and we describe the developmental processes by which people come to view social groups as shaping moral obligation. We conclude with suggestions about how this account can inform the study of social cognitive development more broadly, as well as how it can be used to promote positive moral socialization.