Separation-individuation and attachment theories are compared and assessed in the context of psychoanalytic developmental theory and their application to clinical work. As introduced by Margaret Mahler and John Bowlby, respectively, both theories were initially regarded as diverging from traditional views. Separation-individuation theory, though it has had to be corrected in important respects, and attachment theory, despite certain limitations, have nonetheless enriched psychoanalytic thought. Without attachment an infant would die, and with severely insecure attachment is at greater risk for serious disorders. Development depends on continued attachment to a responsive and responsible caregiver. Continued attachment to the primary object was regarded by Mahler as as intrinsic to the process of separationindividuation. Attachment theory does not account for the essential development of separateness, and separation-individuation is important for the promotion of autonomy, independence, and identity. Salient historical and theoretical issues are addressed, including the renewed interest in attachment theory and the related decline of interest in separation-individuation theory.M argaret S. Mahler and John Bowlby both had personal motivations for their research into the relatively unexplored problems of separation and attachment. Both had experienced maternal insensitivity and rejection. While Mahler's work was acceptable in most quarters and was frequently referred to in the psychoanalytic literature of her day, in some psychoanalytic institutes she was regarded as quite controversial. Some analysts were skeptical about the validity or value of her conceptualization of separation-individuation. Nevertheless, during Mahler's lifetime separation-individuation theory gained a prominent Clinical Professor of Psychiatry and Training Analyst,