Metabolism of oxygen, while central to life, also produces reactive oxygen species (ROS) that have been implicated in processes as diverse as cancer, cardiovascular disease, and aging. It has recently been shown that central nervous system stem cells1, 2 and hematopoietic stem cells and early progenitors3-6 contain lower levels of ROS than their more mature progeny and that these differences appear to be critical for maintaining stem cell function. We hypothesized that epithelial tissue stem cells and their cancer stem cell (CSC) counterparts may also share this property. Here we show that normal mammary epithelial stem cells contain lower concentrations of ROS than their more mature progeny cells. Congruently, subsets of CSCs in some human and murine breast tumors contain lower ROS levels than corresponding non-tumorigenic cells (NTCs). Consistent with ROS being critical mediators of ionizing radiation-induced cell killing7, 8, CSCs in these tumors develop less DNA damage and are preferentially spared after irradiation compared to NTCs. Lower ROS levels in CSCs are associated with increased expression of free radical scavenging systems. Pharmacologic depletion of ROS scavengers in CSCs significantly decreases their clonogenicity and results in radiosensitization. These results indicate that, similar to normal tissue stem cells, subsets of CSCs in some tumors contain lower ROS levels and enhanced ROS defenses compared to their non-tumorigenic progeny, which may contribute to tumor radioresistance.