Myelodysplastic syndromes (MDS) comprise a spectrum of clonal stem cell disorders which are currently defined according to the classification scheme of the revised 2008 WHO classification but which may be further refined in the future. The clinical presentation is often characterized by unexplained isolated or multiple peripheral blood cytopenias resulting in anemia, bleeding events or increased susceptibility to infections. The generally hypercellular, but rarely hypocellular and occasionally fibrotic bone marrow shows dysplastic features in ≥ 10 % of cells of at least one of the hematopoietic lineages. These features and enhanced apoptosis, stem cell senescence and immunologic dysregulation result in ineffective hematopoiesis. Diagnostics in MDS relies on complementary consideration of hematological, morphological and cytogenetic/molecular parameters. Methods include marrow and peripheral blood cytology, cytogenetics, fluorescence in situ hybridization (FISH), trephine bone marrow biopsy examination, immunophenotyping and the evaluation of molecular markers by established and new techniques. Mutations affecting growth factor receptors, cell cycle and apoptosis regulators, intracellular signaling, transcription factors, epigenetic regulation and the splicosome are involved in MDS pathogenesis and progression.