Attention-deficit hyperactivity disorder (ADHD) is a behaviorally defined diagnosis. Despite the fact that neuropsychological tests have typically been used successfully to investigate the functional neuroanatomy of ADHD in neuroimaging research paradigms, these tests have been of surprisingly limited utility in the clinical diagnosis of the disorder. This article examines this paradox by reviewing the characteristics of the Diagnostic and Statistical Manual of Mental Disorders diagnosis versus neuropsychological nomenclature, by reviewing the assumptions about etiologies for ADHD and by demonstrating how an emerging dimensional approach to diagnostic assessment can be combined with large-scale brain network studies to enhance the role of neuropsychological evaluation within clinical settings. This selective topical review is intended to arm practicing neuropsychologists with knowledge of new ideas, theories, and methods related to the causes of ADHD to prepare them for meaningful advances in understanding and assessing the disorder that are possible during the next decade.