This chapter considers relations between adult siblings, measured by kin support, emotional closeness, practical help, financial support and contact frequency. Prior studies have indicated that sibling attachment is created early in childhood through certain cues, including coresidence and shared maternal care, meaning that increased time siblings have spent together during childhood and maternal perinatal association (i.e. seeing one’s own mother nurse a newborn baby) tend to influence adult sibling relations. Also, parental unequal treatment experienced in childhood or adolescence can be remembered for a long time and it may shape sibling ties in adulthood. In addition, it has been detected that sister-sister pairs are closer compared to other sibling constellations; increased age difference between siblings predicts decreased contact and support between sisters and brothers; full siblings are closer to one another compared to half or step siblings; and longer geographical distance between siblings is associated with decreased probability of contact and support. These prior findings are discussed with reference to evolutionary, social psychological and sociological theories about sibling altruism. At the end of the chapter, limitations of existing studies are presented and themes for future investigations are introduced.