A comprehensive, empirically based review of the published studies addressing neuropsychological performance in adults diagnosed with attention-deficit/hyperactivity disorder (ADHD) was conducted to identify patterns of performance deficits. Findings from 33 published studies were submitted to a meta-analytic procedure producing sample-size-weighted mean effect sizes across test measures. Results suggest that neuropsychological deficits are expressed in adults with ADHD across multiple domains of functioning, with notable impairments in attention, behavioral inhibition, and memory, whereas normal performance is noted in simple reaction time. Theoretical and developmental considerations are discussed, including the role of behavioral inhibition and working memory impairment. Future directions for research based on these findings are highlighted, including further exploration of specific impairments and an emphasis on particular tests and testing conditions.