Attention-deficit hyperactivity disorder ( ADHD) is characterized by pervasiveand impairing symptoms of inattention, hyperactivity, and impulsivity, being oneof the most common neuropsychiatric disorders with childhood onset. Since thelatter part of 1990’s, it has been hypothesized that ADHD symptoms are caused byneuropsychological dysfunction, namely executive dysfunction, specifically behavioraldysinhibition. Dopaminergic dysregulation in dorsolateral prefrontal cortex (DLPFC)is thought to be involved in these dysfunctions. But recent studies indicate ADHDsymptoms may also occur from serotonergic dysregulation. In this case, oppositionaldefiant disorder (ODD) and conduct disorder (CD) are highly comorbid with ADHD,and SSRI can have more effect on the symptoms than metylphenidate hydrochlorine(MPH) or amphetamine. In the present review, the author looked into extant evidenceswith regard to “impulsivity” as multifaceted construct, and concluded that, while pureADHD may be caused by behavioral dysinhibition ( impulsivity without emotionalurgency), ADHD with ODD or CD may be caused by impulsivity with emotionalurgency.