Pervasive developmental disorders (PDD) encompass a heterogeneous group of children with deficits of verbal and non-verbal language, social communication, and with a restricted repertoire of activities or repetitive behaviours. The frequency in general population is considered 27.5/10,000. In this study, we analyzed the clinical and genetic aspects of Autism, Asperger Syndrome, PDD Not Otherwise Specified, Rett Syndrome and Childhood Disintegrative Disorder. We analyzed clinical, behavioural and neuropsychological features. We revised different medical genetics associated conditions and divided the genetics aspects of pervasive developmental disorders into two groups: Syndromic forms (around 20%) and non syndromic forms (currently proposed to be 80%). The early recognition of pervasive developmental disorders and the diagnosis of specific associated syndromes allow early therapy, correct genetic counselling, and follow up anticipating possible complications related to the entity. Finally, although the genetic bases of autism have not yet been identified, the following candidate genes have been proposed: 15q, 2q, 17q, 7q, 12q, and X related genes, among others; which are analyzed in this study and will allow a better understanding of these disorders in the future.