Autistic disorders are heterogeneous. Affected individuals show impairments in communication and social interaction, as well as stereotypic, repetitive behaviour and special interests. The majority of autistic disorders are genetic in origin. The current article presents an overview of cytogenetic findings, as well as of results of molecular genetic linkage and association studies. Important differential diagnoses will be described. The results of genetic studies are especially relevant with regard to genetic counselling for affected families.