We entered the 20th century with only meager understanding of the erythrocyte. We leave this century with a relatively detailed understanding of the metabolism of the erythrocyte, the structure of its membrane, and the basis of genetic disorders that lead to its early demise in hemolytic anemia. Among the immune hemolytic disorders, the conquest of Rh hemolytic disease is one of the important clinical achievements of this century. Hereditary disorders of the membrane generally cause shape changes, such as spherocytosis or ovalocytosis. Paroxysmal nocturnal hemoglobinuria is the result of an acquired (somatic) mutation of PIG-A, an X-linked component of the glycosylphosphatidylinositol (GPI) anchor. Red cell enzyme deficiencies cause hereditary nonspherocytic hemolytic anemia. The mutations that cause the more common of these deficiencies are now well understood at the DNA level. Although much progress has been made, much is still to be learned. In particular, management of both acquired and hereditary hemolytic anemias is still very unsatisfactory. Often the only decision that can be made is whether to perform a splenectomy. In the future it is to be hoped that the knowledge that has been gained about these disorders in this century will make available better therapy to our patients in the next.