Two genes, RHD and RHCE, encode the antigens of the RH blood group system. The clinically most important antigen D is determined by the presence of a functional and grossly normal RHD gene. About 18% of Europeans do not express an antigen D, most often but not always caused by the RHD gene deletion. Rhesus negative phenotypes in Africans are caused by the RHD gene deletion, the RHD pseudogene RHD psi, and the Cde(s) allele. About 1% of Europeans carry RHD alleles with aberrant structures encoding for diminished D-immunoreactivity. In Africans the frequency of aberrant RHD alleles is much higher. Aberrant RHD alleles encode partial D, some of which were dubbed D categories, and weak D. Since we defined the molecular basis of the RHD deletion, a specific detection of heterozygous carriers became feasible.