Although the five-part diagnostic criteria of the Diagnostic and Statistical Manual of Mental Disorders Fourth Edition, Text Revision (DSM-IV-TR) for attention-deficit/hyperactivity disorder (ADHD) are behavioral and descriptive in nature, this condition has increasingly been defined as a disorder resulting from impaired behavioral inhibition leading to executive function deficits. Recent research, particularly involving the Planning, Attention, Simultaneous, Successive (PASS) theory offers an understanding of the intellectual and neuropsychological processes implicated in ADHD. We provide an overview of ADHD as a neuropsychological condition; reviews of research on the PASS theory, which provide a process-based understanding of ADHD; and recommendations for assessment and intervention. The research base summarized here provides support that ADHD have a distinctive profile of PASS processes that is consistent with the cognitive nature of their disorder and that researchers have shown increased academic performance when children are taught to better use planning processes when completing academic tasks. C 2008 Wiley Periodicals, Inc.Billy enters the classroom with his usual flair and high level of energy. He yells, "I'm here!" only to be told by his teacher that he needs to put his belongings away and be seated without disrupting the class. Billy begins to unload his backpack then sees his friend reading a new comic book across the room. As his teacher begins to take attendance, she notices that Billy's backpack is on the floor with crumpled papers spilling out. Billy is out of his seat reading the comic book with his friend. When the teacher reminds Billy again that he is not following the morning routine, he quickly tries to organize his belongings and get to his seat. A few minutes later, Billy talks out of turn in the middle of the teacher's morning announcements to inform the class that he has a new puppy. Even though Billy is enthusiastic about being part of the classroom, he has a difficult time controlling his energy and planning his actions. Why does Billy act this way? Billy has attention-deficit/hyperactivity disorder (ADHD). Despite how much Billy wants to, it is difficult for him to follow classroom rules. But beyond the diagnosis of ADHD and the description that Billy is impulsive, inattentive and hyperactive, what specific intellectual process does he struggle with leading to this pattern of behavior?