Attention Deficit Hyperactivity is a childhood-onset disorder that can persist into adult life. Traditional family, twin and adoption studies have shown that ADHD defined both categorically and dimensionally is familial and heritable. Twin studies are now being used to examine ways of defining the ADHD phenotype, to investigate gender differences, the effects on genes on continuity and comorbidity and to consider gene-environment interplay. Molecular genetic findings on ADHD have mainly arisen from functional candidate gene association studies and a number of pooled and meta-analyses have now been conducted. There is consistent evidence of association between ADHD and a dopamine D4 receptor gene VNTR and a dopamine D5 receptor gene microsatellite marker. More recent evidence from different studies and a pooled analysis suggests that conduct problems in those with ADHD is influenced by the COMT val158/108 met variant. Linkage studies suggest that there are no genes of moderate effect size and findings from large scale whole genome association studies are currently awaited. Overall the evidence to date, suggests that examining gene-phenotype links and testing whether gene variants have modifying effects on the ADHD phenotype are important. The contribution of gene-environment interplay (G x E) to psychopathology is becoming increasingly recognised, although for ADHD little is known on causal environmental risk factors.