Although recognized in a multitude of references in psychiatric literature, the importance of loneliness in human behavior has been underestimated. Clinical and experimental evidence, however, seems to point out that loneliness plays a basic part in psychopathological processes, undergirding in some fundamental way most, if not all, forms of psychopathology. Loneliness is here defined as the feeling of no relationship and is distinguished from aloneness. Aloneness is often constructive, whereas loneliness is destructive. Apparently, loneliness is related to the unfulfillment of a need for intimacy instinctual in nature, and in that way relates to maternal absenteeism. The implications of these considerations for psychopathology and psychotherapy are discussed and the hypothesis formulated that the fulfillment of the need for intimacy may constitute a basic factor in human relations which could account for much of what is “therapeutic” in psychotherapy.