Schizophrenia is a common chronic and disabling brain disease of unknown etiology, pathogenesis, and mechanism. Whether schizophrenia represents a single disorder of markedly variable expression or a family of clinically related disorders is unclear. Genetic factors, most likely multiple genes of modest effect, play a major role in its etiology, but an environmental "second hit" may be necessary for clinical expression. The inherited biological susceptibility to schizophrenia is probably expressed clinically as nonpsychotic abnormal personality traits, plus numerous biological markers (cognitive, anatomical, and psychophysiological) that are all found significantly more commonly in the population than is schizophrenia. Neuropathological studies suggest that schizophrenia may be a neurodevelopmental disorder, characterized by reduced neuropil, with no evidence for ongoing cell death. A hypothesized mechanism for these changes involves cell migrational abnormalities occurring in the fetal brain. Schizophrenia is also distinguished by biochemical abnormalities involving the dopamine, GABA, and glutamate systems, and NMDA and nicotinic receptors. Structural and functional brain imaging studies suggest both global and regional abnormalities as well as "disconnections" of specific cerebral circuits. Despite the lack of knowledge regarding pathophysiology, there are reasonably effective treatments for schizophrenia. As the neurobiology of the disorder is unraveled, more effective, targeted treatments will become available.