The frontiers of schizophrenia are being increasingly challenged from several directions. In addition to ongoing debate as to divisions between schizophrenia and disorders of the schizophrenic spectrum, including schizotypal personality disorder and schizophreniform disorder, it has been suggested that obsessive-compulsive disorder might overlap phenomenologically with schizophrenia. There has been a long debate around the relationship of schizophrenia to affective disorders, particularly bipolar and schizoaffective disorder. The evidence suggests that although schizotypal personality and schizophreniform disorders are not homogeneous syndromes, they are related to or represent milder forms of schizophrenia. Obsessive-compulsive disorder seems to involve pathology in many of the same regions as observed in some patients with schizophrenia, which may account for the significant incidence of obsessive-compulsive symptoms in a subset of patients with schizophrenia. Despite similarities between schizophrenia and bipolar disorder, significant differences extend across suggested causes, phenomenology, and pathophysiology. These findings support the current conceptualization that the two disorders represent distinct disorders, probably with heterogeneous causes, rather than the ends of a spectrum of symptoms comprising a single syndrome. Schizoaffective disorder likely is made up of patients from the schizophrenic and bipolar cluster of illnesses. The long-standing debate as to the boundaries of schizophrenia is ultimately must await the eventual further elaboration of the underlying causes of schizophrenia and other psychotic disorders.