Genetic factors are responsible for up to 40% developmental disability cases, such as global developmental delay/intellectual disability (GDD/DI). The American and more recently the European guidelines on this group of diseases state that genetic testing is essential and should become a standardized diagnostic practice. The main arguments for the necessity of implementing such a practice are: (1) the high prevalence of developmental disabilities (3% of the population); (2) the high genetic contribution to this type of pathology; (3) insufficient referral for genetic consultation. In an attempt to address these issues, the purpose of this paper is to present the genetic etiology of global developmental delay / intellectual disability with emphasis on the need to implement a genetic testing protocol for the patients with GDD/DI, as indicated by the current guidelines. Chromosomal abnormalities and fragile X syndrome are the most frequent causes of developmental disabilities and the techniques employed to detect such genetic disorders should be used as first line investigations of GDD/DI.