Kin relationships are traditionally defined as ties based on blood and marriage. They include lineal generational bonds (children, parents, grandparents, and great-grandparents), collateral bonds (siblings, cousins, nieces and nephews, and aunts and uncles), and ties with in-laws. An often-made distinction is that between primary kin (members of the families of origin and procreation) and secondary kin (other family members). The former are what people generally refer to as “immediate family,” and the latter are generally labeled “extended family.” Marriage, as a principle of kinship, differs from blood in that it can be terminated. Given the potential for marital break-up, blood is recognized as the more important principle of kinship. This entry questions the appropriateness of traditional definitions of kinship for “new” family forms, describes distinctive features of kin relationships, and explores varying perspectives on the functions of kin relationships.