Myelodysplastic syndromes (MDS) are a heterogeneous group of bone marrow disorders that affect mostly the elderly and have a variable probability of progression to acute leukemia. The diagnosis of MDS rests largely on a critical morphologic review of blood and bone marrow slides, with careful correlation with other clinical and essential laboratory data, including cytogenetics. This article discusses the epidemiology and clinical and pathologic features of MDS and pertinent diagnostic and prognostic classifications, with a brief overview of treatment options. Other considerations in the differential diagnosis are also briefly outlined.