The hematopoietic system plays roles that are crucial for survival of the host: delivery of oxygen to tissues, arrest of accidental blood leaking from blood vessels, and fending off invading microbes by humoral, cell-mediated, and phagocytic immunity. Today several growth factors are available to the clinician as treatments for various pathologic perturbations of hematopoiesis. Their use has also advanced our capacity to safely deliver cancer chemotherapy. Hematopoietic growth factors have become an everyday tool for the practicing oncologist. Here, experts review the biological interactions between hematopoietic progenitors, growth factors and cytokines, and the stromal micro-environment that have provided the scientific basis for our current understanding of hematopoiesis. In subsequent sections the pathophysiology of neutropenia, anemia and thrombocytopenia are presented, along with discussions of the clinical challenges these pose for the practicing oncologist and hematologist and the roles of various hematopoietic growth factors in therapeutic strategies for these entities.