In describing the various forms of myeloproliferative disease in which myelofibrosis develops, it becomes clear that certain cellular and biochemical abnormalities are shared by all forms of reactive myelofibrosis. Of possible etiologic significance are megakaryocytosis, monocyte-macrophage hyperplasia and activation, altered immunologic status, and proliferation of mast cells and basophils. The mechanisms by which these abnormalities could mediate fibrosis and the methodology that makes assessment and monitoring of fibrosis possible during the course of myeloproliferative disease are discussed in the subsequent contributions to this volume. Opportunities for further study and elucidation of the pathogenesis of myelofibrosis abound and should stimulate the generation of new knowledge that has direct relevance to the management of myeloproliferative disease.