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In this review, we focus on those studies using brain imaging modalities to examine children and adults with attention-deficit hyperactivity disorder (ADHD). The conceptualization of attention that frames our review stresses the adaptive function of attention systems in regulating and orchestrating operations of multiple processing systems to satisfy multiple and sometimes inconsistent goals, and to maintain behavioral coherence. More specifically, we postulate that attention should be conceptualized as consisting of multiple control systems, each of which is associated with a different repertoire of behaviors. A reasonable starting point is the posterior and anterior attentional systems proposed by Posner and associates. Recent functional brain imaging studies support such a notion, with perceptual selection involving the posterior system and executive function engaging distinct anterior frontal regions. To date, the morphometric studies as well as more recent functional imaging studies suggest that subjects with ADHD differ from controls primarily in more anterior brain regions as well as in anterior-striatal systems. Studies now in progress offer the promise of examining neural systems throughout the brain in individuals with ADHD, neural systems involving both anterior as well as posterior attentional systems. Although attention as a construct and ADHD as a disorder were historically described for the first time in the mid-to-late 19th century, it is now apparent that, despite the great progress within the last 4 decades, a full understanding of the neurobiology of ADHD must await the 21st century. MRDD Research Reviews 1999;5:185–190. © 1999 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]