Autism Spectrum Disorder (ASD) is a common neurodevelopmental disorder causing significant lifetime morbidity. Recent advances in molecular genetics research highlighted the contribution of genetic risk factors to increasing genetic susceptibility to ASD. This review outlines the evidence supporting the role of genetic risk factors based on clinical genetic studies that have highlighted relatively strong heritability for ASD. Also discussed are the converging findings from molecular genetic studies showing evidence for the role of rare genetic risk factors, such as copy number variants (CNV) and rare sequence variants, in causing autism in a small proportion of individuals with ASD. Strong evidence for the contribution of common genetic variation to increased risk of ASD will likely require much larger samples as has been demonstrated in other complex genetic disorders. Application of bioinformatic approaches to data emerging from ASD molecular genetics studies is increasingly informing on the biology of the condition, and will ultimately lead to improved diagnostics and therapeutics.