Advances in oxidative stress and redox signaling have led to improved definitions for considerations in health and disease. Oxidative stress is defined in a pathologic sense, while physiologic oxidant production is termed oxidative eustress. Redox biology embraces the continuum of oxidation–reduction reactions in normal biology and pathology, including non-enzymatic as well as enzymatic reactions. This broader view recognizes that an individual has continuous environmental interactions that have major impact on health and disease.