Attention-deficit/hyperactivity disorder (ADHD) is a common developmental disorder often associated with other developmental disorders including speech, language, and reading disorders. Here, we review the principal features of ADHD and current diagnostic standards for the disorder. We outline the ADHD subtypes, which are based upon the dimensions of inattention and hyperactivity. These serve as the phenotype for ADHD. Current nomenclature implies a deficit in the cognitive construct of attention, and this has taken researchers on an extended inquiry into several potential endophenotypes underlying ADHD, in particular executive function and its subcomponents. We review this literature and then delve into the neurobiology of ADHD. This research suggests that the corticostriatal system is a strong candidate system in the etiology of ADHD, in part because of the dopaminergic system, which is known to play a role in the disorder. We present this system as an important contributor to the comorbidity of ADHD with other developmental disorders, especially language disorder.