In the critical care setting, nurses frequently care for patients with acute and chronic diseases that affect multiple body systems. Many of these medical conditions have been associated with an imbalance between oxidizing chemicals called free radicals and antioxidants. Free radical damage is now assumed to be a contributing factor in all major diseases. In order to provide the most current and comprehensive care, critical care nurses need to be well informed about how free radicals cause damage and the antioxidant compounds that neutralize their destructive effects. This article provides an overview of oxygen free radicals and antioxidants and how they impact different clinical illnesses familiar to critical care nurses.