Myelodysplastic syndromes (MDS) are a heterogeneous group of clonal hematopoietic diseases with inefficient hematopoiesis. Diagnosis is based on the persistence of at least one cytopenia for more than 6 months, together with cytological and cytogenetic bone marrow abnormalities, a variety of other diagnosis being discarded. Most patients will die from the consequences of cytopenias. These diseases can progress to acute leukemia. The last WHO classification (2008) distinguishes 6 MDS types. An International Prognostic Scoring System (IPSS) based on the percentage of blast cells in the bone marrow, the cytogenetic abnormalities and the number of cytopenias completes the WHO classification. The patient care depends of the disease severity, cytogenetic abnormalities, age and performance status. Supportive care is required in all cases. In low risk patients, drugs are possibly used to correct cytopenias whereas in high risk patients, the treatment aims to increase survival through induction of a complete response. MDS classification may become molecular in the coming years, based on the genetic abnormalities that lead either to apoptosis of blood cell progenitors in the bone marrow or the accumulation of blast cells that precedes acute leukemia.