Loneliness has long been recognized as a common human problem that causes great psychological distress. Scientific studies on this topic, however, have been slow to emerge. Empirical research on loneliness began in earnest in the 1980s, when psychometrically sound measures of the construct were developed. Initially, research was conducted mainly on college students and the elderly. As the age range examined was extended downward to include childhood and adolescence and some efforts were made to cover adulthood as well, a life-span approach to loneliness has emerged gradually. This entry covers the definition and measurement of loneliness, the theoretical attempts to identify the origins of loneliness in various stages of the life span, and the empirical efforts to describe age trends in loneliness.