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.