Impaired platelet production may result from four mechanisms: 1) marrow hypoplasia, 2) ineffective thrombopoiesis, 3) aberration of thrombopoietic regulation and 4) hereditary thrombocytopenia. Megakaryocytic hypoplasia(reduced numbers of megakaryocytes) is seen in a wide variety of disorders including aplastic anemia, radiation, drugs such as chemotherapeutic agents, infiltrative neoplasms of the marrow, acquired amegakaryocytic thrombocytopenic purpura, paroxysmal nocturnal hemoglobinuria(PNH), and Fanconi's anemia. Ineffective thrombopoiesis(normal to increased numbers of megakaryocytes) is recognized in megaloblastic anemia and myelodysplastic syndrome. Disorders of thrombopoietic control are uncommon and seen in cyclic thrombocytopenia and thrombocytopenia with absent radii. Hereditary thrombocytopenia is overall a very rare disorder, and may be inherited as an X-linked recessive trait, an autosomal dominant trait, or autosomal recessive trait. It is important to recognize hereditary thrombocytopenia because clinically they resemble ITP, but the patients do not respond to steroid treatment or splenectomy.