This video reviews the clinical and pathologic features in the two major forms of acquired bone marrow failure leading to pancytopenia, aplastic anemia and paroxysmal nocturnal hemoglobinuria, as well as acquired uni-lineage disorders such as pure red cell aplasia, agranulocytosis, and amegakaryocytic thrombocytopenic purpura. Fanconi's anemia is also discussed as a paradigm of congenital bone marrow failure.