Aims: Over the past decade, genetic tests have become available for numerous heritable disorders, especially those whose inheritance follows the Mendelian model. Autism spectrum disorders (ASDs) represent a group of developmental disorders with a strong genetic basis. During the past few years, genetic research in ASDs has been successful in identifying several vulnerability loci and a few cytogenetic abnormalities or single-base mutations implicated in the causation of autism. Method: In this study the literature was reviewed to highlight genotype-phenotype correlations between causal gene mutations or cytogenetic abnormalities and behavioural or morphological phenotypes. Results: Based on this knowledge, practical information is offered to help clinicians pursue targeted genetic testing of individuals with autism whose clinical phenotype is suggestive of a specific genetic or genomic aetiology. Interpretation: Comprehensive research into the molecular mechanism of autism is required to aid the development of disease-specific targeted therapies. In order to transfer this recently acquired knowledge into clinical practice, it is critical to define a set of phenotypic inclusion criteria that must be met by affected probands to justify their enrolment in a specific genetic testing programme.