The clinicopathologic heterogeneity of myelodysplastic syndromes (MDS) is driven by diverse, somatically acquired genetic abnormalities. Recent technological advances have enabled the identification of many new mutations, which have implicated novel pathways in MDS pathogenesis, including RNA splicing and epigenetic regulation of gene expression. Molecular abnormalities, either somatic point mutations or chromosomal lesions, can be identified in the vast majority of MDS cases and underlie specific disease phenotypes. As the full array of molecular abnormalities is characterized, genetic variables are likely to complement standard morphologic evaluation in future MDS classification schemes and risk models. Expected final online publication date for the Annual Review of Pathology: Mechanisms of Disease Volume 8 is . Please see http://www.annualreviews.org/catalog/pubdates.aspx for revised estimates.