The 2001 World Health Organization (WHO) classification of hematopoietic and lymphoid neoplasms categorized “the leukemias” into two major groupings—myeloid and lymphoid. Myeloid neoplasms, which are the primary focus of this chapter, include acute myeloid leukemia (AML), myelodysplastic syndromes (MDS), and myeloproliferative neoplasms (MPN). Lymphoid neoplasms are mostly reviewed as part of non-Hodgkin lymphoma in Chapter 40 of this volume, although descriptive patterns and selected etiologic studies are briefly discussed in this chapter because of historical trends. Worldwide, leukemias are ranked eleventh among all cancer types, comprising approximately 2.5% of all malignancies. Exposure to ionizing radiation and certain chemical carcinogens (e.g., cytotoxic chemotherapy, benzene, formaldehyde) are the most consistently associated risk factors for MDS and/or AML. Radiation has been linked with CML, and cigarette smoking with AML. Fewer risk factors have been identified for MPNs. Some evidence implicates increased risks of AML in rubber workers, farmers, and other agricultural workers.