Loneliness is defined as the feeling that one’s social relationships are deficient in some important way, either quantitatively or qualitatively. Results from previous research suggest that loneliness is both a consequence and an antecedent of social relationships and relationship goals. It is still an open question, however, to what extent the experience of loneliness remains stable as people grow older. The aim of the present dissertation is to combine previous research on loneliness and adult development by examining how the average level, social antecedents and social consequences of loneliness change with increasing age. Specifically, I address three major research questions in this dissertation (1) Does the average level of loneliness change between midlife and old age?; (2) Do the social antecedents of loneliness change with age?; and (3) Do the socio-motivational consequences of loneliness change with age? To answer research questions 1 and 2 I used data from the German Ageing Survey (DEAS), a cohort-sequential and nationally representative study of community-dwelling adults in the second half of life (40 years and older) living in Germany. To analyze the socio-motivational consequences of loneliness (research question 3) I used data from a diary study and two experimental studies with young, middle-aged and older adults in Germany. The results of this dissertation show that the average level of loneliness remains relatively stable between midlife and old age. Moreover, the findings suggest that age-related changes in emotional qualities of the social network include both gains (e.g. a reduced number of distressing relationships) and losses (e.g. a reduced satisfaction with friendships). While poor emotional quality of social relationships was equally predictive of loneliness between midlife and old age, the absence of a romantic partner appeared to be less straining and less relevant as an antecedent of loneliness as people grew older. The results regarding the socio-motivational consequences of loneliness indicate that a temporarily heightened level of loneliness may amplify people’s motivation to avoid negative social experiences. There was no indication that the immediate socio-motivational consequences of loneliness differ between young, middle-aged and older adults. Taken together, the findings of this dissertation suggest that age-related stability in loneliness from midlife into old age may reflect changed relationship goals rather than improved relationship quality. Moreover, the results provide first evidence suggesting that the immediate socio-motivational consequences of loneliness may be relatively stable across the adult life span. Further studies are needed to understand how both stable inter-individual differences and intra-individual changes in a person’s relationship goals contribute to the development of loneliness across the adult life span. Specifically, it is an important open question to what extent a flexible adaption of relationship goals may be both beneficial and damaging to a person’s social well-being. It can be concluded that both researchers and practitioners dealing with loneliness should pay greater attention to the opportunity that factors in the emergence and maintenance of loneliness may vary over persons and time.