The objective of this review is to summarize recent data on the genetics of autism, highlight the evidence for genetic heterogeneity and extend the implications of these findings for the identification of susceptibility genes in this disorder. Family studies have shown that autism runs in families and twin studies indicate that the basis of that familial aggregation is genetic. As a result the prospects for the identification of susceptibility genes using either linkage or association studies are quite good. However, recent evidence is accumulating suggesting that the disorder is genetically heterogeneous; higher functioning individuals with autism may arise from separate genetic mechanisms that lower functioning ones. If true, this will make the detection of linkage and association much more difficult.