The chapter addresses loneliness as the subjective experience of a discrepancy between the desired and the achieved personal network of relationships. Social isolation concerns the objective characteristics of a small personal network. A small or not optimally functioning network of contacts with children, other kin and non-kin forms a risk of loneliness. Other factors are a person’s relationships standards, age, gender, educational level, health, migrant status, social skills and self-esteem. The likelihood of being lonely increases with age, and often loneliness persists over time. Characteristics of the societal context, such as inequality, demographic composition, and prevailing values help to understand the level of social isolation and loneliness in varying countries of the world. Research on loneliness is expanding to non-western countries. Separate attention is given to emotional and social loneliness types, and to loneliness measuring instruments. Theoretical approaches to loneliness involve the attachment perspective, the deficit and cognitive discrepancy approaches, and the genetic or evolutionary perspective. Prevention, coping and loneliness interventions are most likely to be successful if they have a clear theoretical understanding of causes and focus on targeted groups. Promising avenues for future research on loneliness include the role of conflict-ridden relationships, and changing patterns of social engagement.