As aerobic creatures, normal living requires that not only are human beings exposed to oxygen but are dependent on oxygen. Humans have evolved mechanisms to cope with living in an aerobic environment; however, modern humans may be more exposed to oxidant stresses. Much indirect evidence implicates reactive oxygen species in diseases such as cancer and atherosclerosis. There are also other diseases that are influenced by oxidative balance, including the normal process of aging. Common environmental factors that could cause oxidative stress include a low intake of dietary antioxidants, a high intake of polyunsaturated fatty acids, and exposure to ozone, ionizing radiation, and cigarette smoke. The recent development of overall measures of oxidant status, such as breath pentane; highly sophisticated measures, such as electron spin resonance and specific measures of base damage to DNA by mass spectrometry; and other methods will allow much more specific data to be collected on the importance of reactive oxygen species in many disease states. Such measures can serve as end points for a variety of studies in experimental animals and humans that will allow for the testing of many potential prooxidant and antioxidant compounds. Other important evidence will be available soon from, for example, large scale chemoprevention trials that are currently under way. As an indicator of the increased interest in oxidant balance, several reference laboratories now have nutritional biochemistry sections that offer measurement of specific free-radical scavenging enzymes. These enzyme measures complement the more routinely available measurements of trace element and antioxidant nutrients.