These two issues of the Seminars in Hematology will provide the physician the necessary knowledge to help make sense of this somewhat confusing array of diseases. The subdivisions of MDS reflect the precision of our techniques of dissection, with morphological and histochemical analyses forming the foundation to identify and subdivide MDS. Although steady refinement has occurred over the last half-century, the basic morphologic technique is unchanged. Cytogenetic analysis, which has been possible since the 1960s and 1970s, should be done at least at initial presentation in all patients to provide refinement of diagnosis and prognosis. FISH is not, at this time, useful as a screening technique. Although the 1990s is an era of rapidly growing knowledge and technical abilities in molecular biology, the use of these techniques in MDS is in its infancy. Very few genes have been identified which are altered in MDS, although many must exist. The molecular assays continue to be cumbersome and impractical to use in the clinical laboratory and remain the domain of the research scientist. Nevertheless, in the future, molecular biology will enable the internist to give each individual a clearer diagnosis and prognosis and may even provide targetted therapies of patients with MDS. At this time the center of management is good supportive care. Some patients, however, will benefit from special interventions, which include the use of growth factors, BMT, and in selected patients, aggressive chemotherapy. Induction of differentiation of the abnormal hematopoietic clone remains only a dream, although some of the differentiation agents may have applicability for their ability to induce apoptosis and prevent growth of the MDS clone of cells. Many of the major advances in our knowledge of cancer developed through the study of hematopoietic malignancy. A lot of these advances are due to the ease of obtaining the abnormal cells. MDS provides an excellent model for studying the progression of cells from their normal to preneoplastic and fully transformed states. A lucid understanding of this progression can form the paradigm for basic science to study neoplastic progression, and the molecular biology techniques used for these studies will be the basic tools used by hematologists and oncologists in the future.