Genome-wide analyses of yeast provide insight into cellular responses to reactive oxygen species (ROS). Many deletion mutants are sensitive to at least one ROS, but no one oxidant is representative of 'oxidative stress' despite the widespread use of a single compound such as H(2)O(2). This has major implications for studies of pathological situations. Cells have a range of mechanisms for maintaining resistance that involves either induction or repression of many genes and extensive remodeling of the transcriptome. Cells have constitutive defense systems that are largely unique to each oxidant, but overlapping, inducible repair systems. The pattern of the transcriptional response to a particular ROS depends on its concentration, and 'classical' antioxidant systems that are induced by high concentrations of ROS can be repressed when cells adapt to low concentrations of ROS.