Attention-deficit/hyperactivity disorder (ADHD) is a highly disruptive childhood-onset disorder that often persists into adolescence and adulthood. Comorbidity with other problems, such as autism, dyslexia and conduct disorder (CD) is very common. Although little is known about the pathophysiology of ADHD, family, twin and adoption studies have shown that it is highly heritable. Whole genome linkage studies suggest there are no common susceptibility genes of moderate effect size. Most published research has been based on functional candidate gene studies. The most consistent evidence for association with ADHD relates to a dopamine D4 receptor (DRD4) gene variable number tandem repeat (VNTR), a dopamine D5 receptor (DRD5) gene microsatellite and a dopamine transporter (DAT1) gene VNTR. In addition, the catechol-O-methyltransferase (COMT) val158/108 met variant has been shown to increase risk for associated antisocial behavior. The first genome-wide association studies (GWAS) of ADHD have been completed and although larger studies are still required to detect common risk variants, novel risk pathways are being suggested for ADHD. Further research on the contribution of rare variants, larger genome-wide association and sequencing studies and ADHD phenotype refinement is now needed.