This entry traces the development of the concept of defense mechanisms from its origins in Freudian clinical practice, following its unfolding in Freud’s work regarding the various ways in which the ego confronts anxieties arising from emotionally threatening experiences, whether internal or external, in neuroses, psychoses, and in the psychopathology of everyday life. In defining the concept and systematizing its occurrences across different forms of psychic functioning, we follow Anna Freud’s work The Ego and the Mechanisms of Defence (1936). We then present the contributions of Melanie Klein (1882–1960), whose observations of young children in analysis brought valuable insights into the early ego and its defenses against anxiety. Finally, we examine briefly some contributions from psychoanalytic work with groups and institutions to the development of the notion of collective defensive strategies against mental pain experienced by group members, regardless of the type of group.