The book presents the world's only twin study involving endogenous psychotic twin subjects based on a polydiagnostic design. Modern, operationalized classification systems (DSM and ICD) are compared with the nosology of Karl Leonhard (1904 - 1988). Detailed case reports of 64 ill twin subjects suffering from psychoses of the schizophrenic spectrum illustrate the diagnostic decisions. The results of the study are discussed in light of all existing twin literature and the latest findings in schizophrenia research. The spectrum of psychoses within the schizophrenic spectrum is clearly not a disease continuum, but consists of various subgroups with very different genetic, somatic, and psychosocial causes.