Myelodysplasia or myelodysplastic syndromes represent a heterogeneous group of bone marrow disorders characterized by dysmaturation, cytopenias, and a propensity for leukemic transformation. Although universally adopted, the French-American-British classification still has several limitations and an inability to categorize all patients. Refinements in morphologic and histologic interpretation in addition to the use of scoring systems may improve diagnostic and prognostic capability. Cytogenetics and molecular genetic abnormalities are providing clues to the fundamental pathogenesis of myelodysplastic syndromes. However, the lesions responsible for initiation or disease progression are as yet unresolved. Although chemotherapy and allogeneic transplantation may be used in selected patients, the mainstay of therapy remains supportive care, with differentiating therapy being largely disappointing so far and the role of hematopoietic growth factors remaining unresolved.