Dissociation is a disturbing psychiatric concept fraught with controversy. It is however encountered in clinical contexts and has to be understood by clinicians. This article based on a Pubmed/Ovid on line research with key words dissociation and borderline personality disorder and other references, describes the clinical aspects of the dissociative phenomena in patients with borderline personality disorder. Psychodynamic hypotheses and neurophysiological data are examined to explain dissociation. Neurophysiological and neuroanatomical variables provided by cerebral imagery controlled studies support hypotheses brought forward. The article concludes with a defence mechanism developed within a context of biological predisposition, deprived psychological development and in reaction to trauma. In conclusion, emerging psychotherapeutic solutions are summarized.