Attention-deficit/hyperactivity disorder (ADHD) is a developmental disorder which effects an estimated 3% to 5% of children. Despite estimates that ADHD persists in 30% to 70% of adults having had the disorder in childhood, ADHD in adulthood remains controversial. This report summarizes current thinking in the diagnosis and etiology of adult ADHD. Most theories posit that ADHD is related to anomalies in frontal lobe function and dopaminergic transmission. However, there is debate as to whether ADHD is a unitary disorder with different manifestations, a syndrome, or multiple disorders. The Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, classifies ADHD into inattention, hyperactivity-impulsivity, and combined subtypes. Although problems with cognition are core ADHD symptoms, self reporting has not been a reliable predictor of neuropsychological test performance. Nevertheless, we suggest that a performance-based diagnosis, including empirically derived, age-sensitive neuropsychological tests, provides the best hope of dissociating ADHD from psychiatric disorders with similar symptoms. We also describe the promise of new neuroimaging technologies, such as functional magnetic resonance imaging, in elucidating the pathophysiology of ADHD and similar psychiatric disorders.