Myelodysplastic syndrome (MDS) is a clonal stem cell disorder characterized by ineffective hematopoiesis, multilineage dysplasia, peripheral cytopenias with normocellular or hypercellular marrow, and susceptibility to leukemic transformation. MDS represents a heterogenous group of disorders with a wide spectrum of clinical, morphological, biologic, and genetic characteristics. MDS is classified according to the World Health Organization criteria and the International Prognostic Scoring System. Risk-adapted treatment strategies have been developed in consideration of the advanced age of patients and to improve the clinical course of the disease. The pathophysiology, cytogenetic/molecular profiles, clinical characteristics and treatment modalities according to the prognostic groups will be described. In the future, a combination of treatment modalities to increase gene reactivation and to take advantage of increased expression of target genes may be critical to improve clinical outcomes. Multiple pathways may be involved in the MDS phenotype, and combination therapies, including novel agents, may be required to make further progresses in the treatment of this disease.