Adult hematopoietic stem cells self‐renew, proliferate, and differentiate forming and maintaining appropriate levels of all the cellular components of blood as well as responding to increased demand. This lifelong process termed hematopoiesis is regulated in part by hematopoietic growth factors, interleukins, and chemokines. In an incredibly short period of time, since the discovery of culture systems and animal models that detected the growth of blood progenitor cells and identified the existence of hematopoietic stem cells in the 1960s, cytokines controlling their function have been identified, cloned, and brought into clinical practice. These agents have had a major impact on patient outcomes and the ability to treat disease. Clinicians now have the tools to modulate erythrocyte, neutrophil, and platelet production enabling treatment of congenital and iatrogenic conditions and harvest of hematopoietic stem and progenitor cells for curative hematopoietic transplantation. In this chapter, hematopoietic cytokines, mimetics, and other agents approved for use in humans are described. The fields of experimental hematology and cytokine research continue to expand and new interleukins continue to be identified. As our knowledge of hematopoietic cell function increases, so will our understanding of the cellular and molecular biology of cytokine actions, and the clinical use of these agents will be refined. The potential application of several newly defined cytokines is discussed as well as the potential for the future development of cytokines that directly regulate hematopoietic stem cell function.