Diagnosis of behavioural disorders has been essentially relied for many years on the diagnostic criteria established by the World Health Organization and the American Psychiatric Association. These criteria are included in the current widely used diagnostic systems ICD-10 and DSM-IV, which in turn are based on multi?axial classification, use almost the same terminology and adopt a categorical perspective. These international classification systems of mental disorders have largely facilitated research as well as communication between specialists in the field of psychopathology. However, their contribution to the understanding of the etiology of psychiatric disorders and their treatment is strongly questioned. These systems do not rely on a specific theoretical framework and the suggested classification is based on the symptoms rather than the causes of a disorder. However, it is often the case that several symptoms may be common in different disorders. For this reason, comorbidity tends to be a rule in psychopathology, rather than exception, and specialists often focus on differential diagnosis. Using the example of Attention Deficit / Hyperactivity Disorder (AD/HD), this symposium presents the deficiencies of the current classification systems and suggests that, instead of following the categorical classification, one should investigate mental problems as spectrum disorders, where the limits of the distinction between symptoms are not any more absolute. This endeavor should adopt a developmental approach and focus on the origins and causal factors of mental disorders.