Genetic analyses which are relevant to plant and animal studies, rather than human populations, have specific limitations in relation to the genetics of schizophrenia. Some reviewers of family studies of schizophrenia have drawn the conclusion that schizophrenia and bipolar disorder do not share the same genetic etiologies whereas others argue that they often do. Considerable effort has been focused on genetic linkage analysis of schizophrenia employing genetic markers in multiply affected families to identify which chromosomal regions harbor susceptibility genes. This approach must take into account the complication of heterogeneity of linkage in which a number of susceptibility genes localized to different chromosomes contribute to the development of schizophrenia. The chapter describes a selection of genes that have been implicated in susceptibility to schizophrenia by cytogenetic, linkage and/or association studies. Twin and adoption studies have shown that the family environment has no influence on the etiology of schizophrenia.