Autism is a pervasive developmental disorder characterised by an impairment in social interaction and in communication, with unusual behaviour. Genetic factors are predominent in autism pathogenesis, in contrast with the environmental factors that would modulate the phenotype. The genetic polymorphism and the phenotypic heterogeneity make the autism a complex disorder to study. Genetic research on families with multiple affected children and biochemical mechanisms studies represent the sources for identifying the susceptibility genes in autism.