Myelodysplastic syndromes (MDS) are clonal disorders of an immature hematopoietic progenitor cell. MDS hematopoiesis is characterized by abnormal progenitor proliferation and impaired cellular differentiation and maturation. As a consequence, the majority of MDS patients have a gradual evolution towards progressive bone marrow failure and/or leukemic transformation. The efforts made by several research groups during the last few years have helped to unravel the complex underlying pathogenesis of MDS. In this review, we will focus on the principal cytogenetic and molecular abnormalities involved in MDS initiation and disease progression. Additionally, we will discuss the influence of the marrow microenvironment on survival and proliferation of hematopoietic progenitors in MDS.