The present theory of friendship is based on a conception of self that regards a central motive to be the person's concern for the well-being and worth of the entity (s) he identifies as self. This concern manifests itself in tendencies to affirm one's sense of individuality, affirm one's more important self-attributes, evaluate one's self positively, and change toward positive self-growth. Friendship involves investments of self in a relationship characterized by the partners' voluntary interdependence and personalized concern for one another. The investment, entailing expenditures of time, personal resources, and personalized concern yields dividends experienced concretely as a partner's self-affirmation value, ego support value, stimulation value, or utility value. Several facets of friendship growth and change are considered, including the degree to which the relationship is difficult to maintain.