The Rh (Rhesus) blood group system is the most complex of the known human blood group polymorphisms. The expression of its antigens is controlled by a two-component genetic system consisting of RH and RHAG loci, which encode Rh30 polypeptides and Rh50 glycoprotein, respectively. Over the past decade, there has been a rapid advance in knowledge of the biochemistry, molecular biology, and genetics of the Rh genes and proteins. The primary structures of D and CcEe antigens have become well understood and the molecular genetic basis of a vast array of phenotype polymorphisms has been delineated. The identification of various molecular defects associated with Rh deficiency syndrome clarifies the nature of the amorph, suppressor, and modifier genes. The observed mutation spectrum defines a basic set of components essential for Rh complex assembly in the erythrocyte membrane. The resulting molecular information, combined with new experimental tools, is helping to dissect the fine structure of Rh antigens in terms of epitope mapping. The discovery of novel Rh homologs in primitive organisms and in nonerythroid tissues opens new avenues of research beyond the scope of erythrocytes and Rh antigens. This review provides an update on the Rh family in antigen expression, phenotype diversity, and disease association.