Autism is a severe psychiatric disorder characterised by deficits in social interaction, disturbed communication and adherence to stereotype routines and interests. Nowadays it is completely clear that this disorder has a biological basis and many observations show strong genetic determination of autism. The importance of genetic factors is supported by frequent association of this disorder with known hereditary diseases or with various chromosomal aberrations, by high concordance of the disorder in monozygotic twins, higher risk for the siblings of autistic patients and also by the frequent occurrence of milder symptoms of the autistic spectrum in more distant relatives. All these findings show that the autistic phenotype results from unfavourable combination of alleles of several genes in interplay with factors of the environment. This model of multifactorial inheritance of autism serves at present as the starting point for the search for predisposing genes in the human genome. The association is tested between autism and alleles of candidate genes selected based on known biochemical and physiological role of their protein products, or based on their location close to recurrent chromosomal rearrangements or in regions identified by whole-genome linkage analyses. Studies of most of these genes have not yielded clear-cut results yet, but the participation of some of them in the aetiology of autism is possible.