Myelodysplastic syndromes (MDS) are a collection of cytogenetically heterogeneous clonal bone marrow (BM) failure disorders derived from aberrant hematopoietic stem cells in the setting of an aberrant hematopoietic stem cell niche. Patients suffer from variably progressive and symptomatic bone marrow failure with a risk of leukemic transformation. Diagnosis of MDS has long been based on morphologic assessment and blast percentage as in the original French-American-British classification. The recently developed Revised International Prognostic Scoring System provides improved prognostication using more refined cytogenetic, marrow blast, and cytopenia parameters. With the advent of deep sequencing technologies, dozens of molecular abnormalities have been identified in MDS.