Attention-deficit/hyperactivity disorder (ADHD) is a common psychiatric condition that affects approximately 5.3% of children worldwide. This disorder is defined by a combination of symptoms of inattention and hyperactivity/impulsivity. Diagnosis is based on impairment in these two domains determining several problems in personal and academic life. Although it is known that genetic and environmental factors are important in ADHD etiology, how these factors influence the brain and consequently behavior is still under debate. There seems to be a consensus in the literature that a fronto-subcortical dysfunction is responsible, at least in part, for the ADHD spectrum. Considering that these brain regions are rich in dopamine (DA), the DA hypothesis has an important role to understand ADHD pathophysiology. The main goal of the present review is to show evidence from different areas that support the idea that dysregulation in the DA system underlies ADHD. We discuss here evidences from animal models, pharmacology, brain imaging and genetics studies.