Despite related controversies and argument, schizoaffective disorder is a crucial mental illness that demands attention and study from both scholars and clinicians. However, most of these discussions are merely debates focused on diagnosis, etiology and genetics of persons with schizoaffective persons. Although there is a long awareness of the uniqueness of schizoaffective disorder, comparing the rich theoretical underpinnings in schizophrenia and mood disorders, the studies of schizoaffective disorder are thin and insufficient. After a long history of conceptual development, these two groups of mental illness are profoundly studied, explained and articulated by biochemical, psychodynamic, cognitive, existential, phenomenological, sociological, cultural and humanistic perspectives. In each perspective, there are numerous accounts, and research in describing persons with schizophrenia or mood disorder. Schizoaffective disorder is increasingly common in various countries around the world; this book tries to capture the situation in defining, understanding, intervening and rehabilitating cliens with schizoaffective disorders in various countries, in particular in Europe and North America, Asia and Australia.