The key features of attachment theory are summarized and the unresolved questions considered in terms of a behavioural control system, measurement of attachment security, qualities of attachment, the role of temperament, transformation of a dyadic quality into an individual characteristic, internal working models, manifestations of attachment post infancy, how one relationship affects another relationship, boundaries of attachment, associations with later functioning, the role of parenting qualities and patterns of caregiving, adaptive value of secure attachment, and disorders of attachment. The clinical implications are discussed in terms of: the need to reject the traditional psychoanalytic theories of development, the patterns of residential care for children, the provision of child care, the assessment of parenting, the effects of parental divorce and family break-up, "maternal bonding" to infants, psychotherapy and disorders of attachment.