Defense mechanisms are mental operations that are involuntary, unconscious, and contribute to reducing internal and external stresses. The concept of defensive organization or style, defined as a set of relatively stable defense mechanisms that are characteristic of the personality of a subject, appears to be a major dimension of personality, from normal to pathological. Studies on defense mechanisms have attracted the interest of clinicians working largely outside of the psychoanalytical field. However, the lack of reliability and validity of the assessment instruments still limits empirical studies of the relationship between defense mechanisms and psychological health and their therapeutic implications.