Recent advances in anatomical, behavioral, and physiological techniques have produced new information about the nature of prefrontal function, its cellular basis, and its anatomical underpinnings in nonhuman primates. These findings are changing our views of prefrontal function and providing insight into possible bases for human mental disorder. A major advance is the recognition that various prefrontal areas are engaged in holding information "on line" and updating past and current information on a moment-to-moment basis. Studies of animals and of cognitive function in normal, brain-injured, and schizophrenic subjects support the theory that a defect in working memory--the ability to guide behavior by representations--may be the fundamental impairment leading to schizophrenic thought disorder.