Myelodysplastic syndromes (MDS) are heterogeneous group of neoplastic clonal stem cell diseases characterized by dysplastic morphological features and clinical bone marrow failure. The FAB (French-American-British) system served as the gold standard for MDS classification for more than two decades. The WHO classification, built on the backbone of FAB classification, is an attempt to further improve the prognostic value of MDS classification as well as establish its clinical utility as a tool to select different treatments. In this article we review the epidemiology, pathogenesis, molecular biology, diagnosis and classification of MDS. We highlight the major differences between the FAB classification and the WHO MDS classification. We discuss in more detail the experience of using the new WHO classification since its publication and review the studies that tried to validate the prognostic value of the new classification or apply it to predict clinical responses to various treatments.