An account is given of attachment theory as a way of conceptualizing the propensity of human beings to make strong affectional bonds to particular others and of explaining the many forms of emotional distress and personality disturbance, including anxiety, anger, depression and emotional detachment, to which unwilling separation and loss give rise. Though it incorporates much psychoanalytic thinking, many of its principles derive from ethology, cognitive psychology and control theory. It conforms to the ordinary criteria of a scientific discipline. Certain common patterns of personality development, both healthy and pathological, are described in these terms, and also some of the common patterns of parenting that contribute to them.