Over the entire life span, social relationships are essential ingredients of human life. Social relationships describe regular interactions with other people over a certain period and generally include a mental representation of the relationship and the relationship partner. Social relationships cover diverse types, such as those with family members, romantic partners, friends, colleagues, as well as with other unrelated people. In general, most of these relationships change in number, contact frequency, and relationship quality during adulthood and old age. For example, both the number of and contact with friends and other unrelated people generally decrease with advancing age, whereas the number of and contact with family members remain rather stable. Relatively little is known about longitudinal changes in the quality of relationships, apart from romantic relationships, because few longitudinal studies have tracked specific relationships. Some explanatory factors, which are discussed in the literature, are (a) motivational changes, (b) reduced time due to work and family demands during adulthood, and (c) resource constraints in older age. Future work on social relationships would benefit from increasingly applying dyadic and network approaches to include the perspective of relationship partners as well as from examining online and offline contact in social relationships, which has already proved important among younger adults.