Schizophrenia, a term that encompasses a diverse and seemingly unrelated class of psychopathologic conditions, has been the object of extensive research. This two-part paper reviews developments in specific areas of schizophrenia research. In Part I the authors discuss research into diagnosis and prognosis. They outline the studies that have been done on the major diagnostic systems including the first-rank symptoms, the flexible system, the New Haven Schizophrenia Index, the Feighner criteria, the Research Diagnostic Criteria, and the Diagnostic and Statistical Manual of Mental Disorders, third edition. They also report on the studies that have compared the utility and reliability of these diagnostic approaches. In Part II, to be published in the May issue, the authors cover the neuropathological studies of the disorder, the genetic research, and the various biological hypotheses of the etiology of schizophrenia.