Myelodysplastic syndromes (MDS) are characterized by peripheral cytopenias in combination with a hyperplastic bone marrow. During the last 15 years, important progress has been made in the understanding of the biology and prognosis of myelodysplastic syndromes. The classification according to the World Health Organization (WHO) includes mainly morphological criteria and is supplemented by the International Prognostic Scoring System (IPSS) which takes cytogenetical changes into consideration when determining the prognosis of MDS. Also MDS after radiotherapy, chemotherapy or chemical exposure must be distinguished from primary MDS. The underlying mechanisms in primary MDS have not yet been established but it is a multistep alteration to the hematopoietic stem cells that include genes involved in cell cycle control, mitotic checkpoints as well as growth factor receptors, secondary signal proteins and transcription factors which gives the cell a growth advantage over its normal counterpart.