Defence mechanisms are mental operations that are involuntary and unconscious and contribute to reduce internal and external stresses. The concept of defensive organisation or style, defined as a set of defence mechanisms relatively stable and characteristic of personality appears to be a major dimension of personality, from normal to pathology. Studies on defence mechanisms have gained the interest of clinicians, largely outside the psychoanalytical field. However, the lack of reliability and validity of the assessment instruments still limits the empirical studies of the relation between defence mechanisms and psychological health as well as of their therapeutic implications.