Hematopoietic stem cells (HSCs) are defined by their ability both to self-renew and to give rise to fresh blood cells throughout the lifetime of an animal. The failure of HSCs to self-renew during aging is believed to depend on several intrinsic (cell-autonomous) and extrinsic (non-cell-autonomous) factors. In this review, we focus on how dysregulation of reactive oxygen species (ROS) and disruptions of genomic stability can impair HSC functions. Recently, it was shown that long-term self-renewing HSCs normally possess low levels of intracellular ROS. However, when intracellular ROS levels become excessive, they cause senescence or apoptosis, resulting in a failure of HSC self-renewal. Repression of intracellular ROS levels in HSCs by treatment with an antioxidant that scavenges ROS can rescue HSC functions, indicating that excess ROS levels are at the root of HSC failure. Products of numerous genes that are involved in either DNA-damage responses or longevity-related signaling contribute to the maintenance of the HSC self-renewal capacity. Further investigations on the molecular mechanisms of ROS regulation and on the manipulation of excess ROS levels could lead to the development of novel therapeutics for hematopoietic diseases, regenerative medicine, and the prevention of leukemia.