This document reviews oxidative stress and antioxidant defense. It summarizes that reactive oxygen species are produced through normal cellular metabolism and environmental factors and can damage cells. At high levels they cause oxidative stress. While low to moderate levels serve physiological functions. Organisms have antioxidant systems like enzymes and molecules to prevent oxidative damage, but these can be overwhelmed during pathology. Oxidative stress contributes to many diseases. The document reviews sources of oxidants and antioxidants in the body as well as their roles in health and disease.