Attention-deficit/hyperactivity disorder (ADHD) is a heterogeneous disorder characterized by inattention, impulsivity, and/or hyperactivity in children and adults. Putative environmental risk factors for ADHD include toxin and prenatal smoke exposure, low socioeconomic status, and parental marital instability and discord. Genetic associations with ADHD have been found in the dopaminergic, serotonergic, and noradrenergic neurotransmitter systems, but findings are inconsistent across studies. Herein, we review studies of gene-environment interactions for ADHD to better understand how genetic and environmental risk factors may contribute to the disorder in a nonindependent fashion, which may account in part for the inconsistent findings on genetic associations. Although evidence of interactions between prenatal substance exposure and the dopamine genes DAT1 and DRD4 was found, findings across studies have been mixed. We discuss these findings and the future directions and limitations of current gene-environment research.