In generic psychiatry there has been increasing interest among scientists for the developmental perspective. However, professionals active in the mental health care of people with intellectual disability (ID) have not shown the same degree of interest. The author of this article, who has had a liberal amount of rewarding experiences with the developmental approach in the field of ID, considers the developmental perspective to be innovative and very useful in psychiatric assessment, diagnosis and treatment of this population. The aim of the article is to stimulate a wider application of the developmental perspective as well as to challenge a professional discussion on this issue. Basic assumptions of the developmental perspective are discussed and assessment tools and methods are described. Results in a case vignette, the advantages of developmentally based assessment are emphasized. Emotional development and personality development are viewed as the developmental components that play an important role in adaptive and maladaptive behaviour as well as in the onset and presentation of psychopathology. It is clear that interpretative insight into the totality of the psychosocial aspects of these individuals cannot only be obtained by measuring the level of cognitive development. A wider frame of mind is needed for unambiguous psychiatric diagnostics. Therefore, a replacement of the three dimensional paradigm (biopsychosocial) by a four dimensional one (bio-psycho-socio-developmental) for the assessment and diagnosis of persons with ID is proposed.