In this very scholarly paper, Bibring traces the development of the theory of the instincts. Freud first postulated two sets of instincts, the sexual and the ego instincts. With the introduction of the concept of narcissism, libidinal aspects of the ego instincts were recognized. Increasing attention to aggressive drives led to their being attributed to the ego instincts. Finally, aggressive drives were recognized as co-existing with libidinal drives in the id; ego instincts were viewed as being derived from both libidinal and aggressive components. The paper concludes with a lengthy discussion of the theoretical advantages accruing from the postulation of a death instinct.