Reactive oxygen species (ROS) have historically been viewed as toxic metabolic byproducts and causal agents in a myriad of human pathologies. More recent work, however, indicates that ROS are critical intermediates of cellular signaling pathways. Although it is clear that dedicated cellular ROS producers such as NADPH oxidases participate in signaling, evidence suggests that mitochondrial production of ROS is also a tightly controlled process, and plays a role in the maintenance of cellular oxidative homeostasis and propagation of cellular signaling pathways. Production of ROS at mitochondria thus integrates cellular energy state, metabolite concentrations, and other upstream signaling events and has important implications in cellular stress signaling, maintenance of stem cell populations, cellular survival, and oncogenic transformation.