The concept of a behavioural phenotype was first introduced by Nyhan (1972) when he described a characteristic behaviour of self-mutilation in Lesch-Nyhan-Syndrome. Later, Flint and Yule (1994) pointed out that a behavioural phenotype is a characteristic pattern of motor, cognitive, linguistic and social abnormalities that is consistently associated with a biological disorder. In some instances the behavioural phenotype may represent a psychiatric disorder, in others there may be behaviours that usually will not be regarded as a psychiatric disorder. Dykens (1995) added that a behavioural phenotype may best be described as the heightened probability that people with a given syndrome will be characterized by certain behavioural sequelae. The implications of this are (i) that there is a certain amount of within-syndrome variability, (ii) that there is total vs. partial specificity of certain behaviours, (iii) that behavioural phenotypes relate to various behavioural domains, (iv) that behavioural phenotypes have indirect effects on the behaviour of caretakers, and (v) that developmental and contextual factors in addition to genetic factors contribute to behavioural phenotypes (Hodapp & Dykens, 2007). The rather recent interest in the concept of behavioural phenotypes has renewed the interest of Child and Adolescent Psychiatry in intellectual disability issues that had been lost for decades. It also increased interest in the genetic origins of mental disorders and behavioural abnormalities with an interaction of two major recent scientific developments in the field. One of these trends is represented by the rapid progress in molecular biology that has led to a better understanding of the genetic mechanisms contributing to psychopathology. The other trend is based on the development of more and more refined instruments for the dimensional assessment of adaptive and maladaptive behaviours which have supplemented the classical categorical approach of assessment in child psychopathology. With this theoretical underpinning, the study of behavioural phenotypes has become an attractive research domain in child and adolescent psychiatry. In addition, there are also practical implications in terms of improving communication among experts and lay-person both on assessment and counselling.