Evidence for the existence of a genetic factor in the etiology of a significant proportion of the people diagnosed as having a schizophrenic disorder is reviewed. It is suggested that whatever is transmitted genetically need not be inherently pathologic and/or pathogenic. It is argued that only people who have certain trait expressions or phenotypes are capable of a schizophrenic decompensation but that these phenotypes, while genetically loaded, are not necessarily pathogenic. An effort is made to show that even those cases in whom genetic factors operate are not homogeneous but represent separate subgroups which differ in their etiopathogenesis. This etiologic heterogeneity, in the development of a characteristic necessary but not sufficient for a schizophrenic decompensation, will almost certainly be associated with differences in the clinical course of the disorder.