Advances in the methodologic study of megakaryocytopoiesis are discussed, and their effect on the understanding of the production of platelets is included. This knowledge has led to a better understanding of disorders of megakaryocytopoiesis and platelet production. Specific diseases discussed are congenital megakaryocyte hypoplasia, acquired amegakaryocytic thrombocytopenic purpura, myeloproliferative disorders, reactive thrombocytosis, megakaryoblastic leukemia, and megakaryocyte and marrow fibrosis.