Attachment theory is based on the joint work of John Bowlby (1907-1991) and Mary Salter Ainsworth (1913- ). Its developmental history begins in the 1930s, with Bowlby's growing interest in the link between maternal loss or deprivation and later personality development and with Ainsworth's interest in security theory Although Bowlby's and Ainsworth's collaboration began in 1950, it entered its most creative phase much later, after Bowlby had formulated an initial blueprint of attachment theory, drawing on ethology, control systems theory, and psychoanalytic thinking, and after Ainsworth had visited Uganda, where she conducted the first empirical study of infant-mother attachment patterns. This article summarizes Bowlby's and Ainsworth's separate and joint contributions to attachment theory but also touches on other theorists and researchers whose work influenced them or was influenced by them. The article then highlights some of the major new fronts along which attachment theory is currently advancing. The article ends with some speculations on the future potential of the theory.