Myelodysplastic syndromes are a group of clonal hematopoietic stem cell disorders in which there is ineffective hematopoiesis. Affected patients are primarily elderly people, but also at risk are certain subsets of patients who have been exposed to antineoplastic chemotherapy and select patients with primary bone marrow failure syndromes such as Fanconi anemia or aplastic anemia. The diagnosis of myelodysplastic syndrome implies a certain probability of leukemic evolution, but the magnitude of this probability varies greatly among patients. Over the years there have been several diagnostic and prognostic classifications of these disorders in an attempt to achieve diagnostic accuracy that is biologically and clinically meaningful. With the aging of the population and with improved survival among patients with cancer, pathologists are likely to confront myelodysplastic syndromes more frequently in their practice. This review discusses the clinical and pathologic manifestations of this group of disorders, major diagnostic and prognostic classification systems, pathogenesis, and select issues that might be clinically relevant in the future.