Human blood group antigens are unique, inherited polymorphisms on the extracellular surface of red blood cells. They have been used as genetically discrete markers of human polymorphism since the discovery of the ABO system in 1900. Since then, many blood group antigens have been identified, the genes cloned, and their biological significance elucidated. Blood group antigens and antibodies play an important role in Transfusion Medicine. In addition, blood groups have provided anthropologists with a tool to study polymorphism in the different peoples across the world and provided geneticists with inherited markers to understand complex mechanisms of linkage and disease inheritance.