ADHD is highly heritable, but environmental factors also play significant roles in disease aetiology and outcome. Genetic and environmental influences are likely to show different types of interplay, with gene–environment interactions (G×E) playing a part. Different models of G×E exist, with the most frequently investigated in ADHD up to the present being the diathesis–stress and differential susceptibility models. The most frequently studied have been monoaminergic genes, often based on a single genetic variant. Only a single genome-wide study has been reported thus far. Environmental factors investigated include prenatal and postnatal risk factors for ADHD, in particular prenatal exposure to smoking or alcohol and aspects of parenting.