Reliable classification and prognostic scoring systems for myelodysplastic syndromes (MDS) are needed to facilitate medically appropriate treatment and management decisions. The French-American-British (FAB) classification scheme for MDS was described in 1982 and has become the reference standard for subsequent MDS classification schemes. The FAB classification system divides MDS into five subgroups, based mainly on morphologic criteria and the percentage of myeloblasts in bone marrow (BM). More recently, the increasing availability of BM cytogenetics, immunologic markers, and molecular genetics has provided important information for staging, prognosis, and treatment of MDS. A World Health Organization panel incorporated this new diagnostic information into a revised classification system that modified the FAB criteria while retaining most of its basic features. The major changes included the creation of additional categories (eg, 5q- syndrome), distinction of unilineage from multilineage dysplasias in the refractory anemias, and subdivision of the heterogeneous refractory anemias with excess blasts into two categories based on BM blast percentage. Additionally, some MDS subtypes were removed or merged with other myeloid disorders into newly created categories. In independent validations, the World Health Organization revisions were shown to provide more-homogeneous subgroups of patients and greater prognostic power compared with the FAB system, although controversies remain. The International Prognostic Scoring System combines blast percentage, karyotype, and number of cytopenias to generate a scoring system that reliably estimates survival and risk of transformation to acute myeloid leukemia for patients with MDS. This universally accepted scoring system is often combined with FAB or World Health Organization morphologic criteria to provide a more complete clinical picture and the most accurate prognostic assessment possible. As more is learned about the pathogenesis of MDS at the molecular level, it is anticipated that these classification and scoring systems will continue to evolve to incorporate the new information.