RH is the most complex of all 29 blood group systems. New discoveries relating to the RHD gene, and an appreciation of its variant phenotypes such as weak D and partial D, have challenged the way that D status is assigned to both blood donors and blood product recipients. This concise review introduces the current concepts of weak D and partial D and how the identification of these variants has influenced the testing methods for the D antigen. We demonstrate how molecular tests of the RHD gene can and should be used in resolving serological discrepancies, in particular in pregnant women.