This chapter provides findings from structural and functional magnetic resonance imaging (fMRI) research focusing on adults with attention-deficit hyperactivity disorder (ADHD). It summarizes existing knowledge from structural and functional neuroimaging research in ADHD brain. Three studies have examined neuroanatomic deficits over a relatively wider range of brain regions, using different methods on the same or overlapping samples. Adults with ADHD showed volume/thickness reductions over a network of frontal and parietal regions involved in attention/executive functions, the orbitofrontal cortex, and the cerebellum. The majority of fMRI studies in ADHD have focused on the neural mechanisms underpinning executive function deficits. The chapter presents the existing research based both on an earlier systematic review of fMRI research in ADHD and an examination of some more recent work in this area. A study has suggested that the commonly observed executive function deficits in ADHD may be partly caused by deficits in more fundamental processes.