Hematopoietic stem cells (HSCs) are characterized by pluripotentiality and a capacity for self-renewal. In order to both maintain a supply of mature blood cells and not to exhaust HSCs throughout the lifespan of the organism, most HSCs remain quiescent and only a limited number enter the cell cycle. In HSCs, the cell cycle is crucially regulated by external factors such as cytokines and interactions with stromal cells and the extracellular matrix (ECM) in the bone marrow (BM) microenvironment. In addition, intrinsic transcription factors expressed in HSCs, including c-Myb, GATA-2, HOX family proteins, and Bmi-1, also control their growth through their effect on gene transcription. In terms of the particular roles in regulation of the cell-cycle, p21WAF1 (p21) and p27KIP1 (p27) were shown to maintain the quiescence of HSCs and of progenitor cells, respectively, thereby governing their available pool sizes. Also, p16INK4A (p16) and p15INK4B (p15) are thought to act as tumor suppressors, since their inactivation and/or deletion are observable in various types of hematologic malignancies. These results make evident that appropriate cell cycle control, particularly at the early stage of stem/progenitor cells, is required for maintaining normal hematopoiesis.