Myeloproliferative diseases (MPD) and myelodysplastic syndromes (MDS) are a heterogeneous group of chronic myeloid disorders diagnosed in over 20,000 patients annually in the United States (Tefferi, 2001). The bone marrow (BM) shows normal or increased cellularity in MDS, but is always hypercellular in MPD. Whereas the percentage of blasts at diagnosis ranges from 1%–20% in MDS, it is normal (< 5%) in early stage MPD. In MDS, there is morphological evidence of BM myeloid dysplasia usually accompanied by cytopenias, while in MPD there is relatively normal myeloid maturation with an overproduction of circulating myeloid elements and an elevated risk of thrombosis. By contrast, acute myeloid leukemia (AML) is characterized by an excess of bone marrow blasts with defective production of mature cells (Gilliland and Tallman, 2002). MPD and MDS are characterized by clonal hematopoiesis and an increased risk of developing acute leukemia, but the frequency of malignant progression and overall prognosis varies greatly between different subtypes. In the past decade, there have been major advances in our understanding of the pathogenesis and treatment of these diseases.