Epidemiological twin studies demonstrate that autism spectrum disorders (ASDs) represent genetic disorders. Subsequent analyses indicate that the causes of ASDs include less common single-gene mutations and chromosomal abnormalities, as well as ASDs caused by multiple interacting genes of weak effect. Genome-wide linkage analysis has identified several susceptibility loci for the ASDs, and positional and functional candidate genes have been identified that appear to represent susceptibility genes for the ASDs. Analysis of additional larger samples and the use of genome-wide association and high-throughput variant detection will lead to the identification of further genes for ASDs.