The role of reactive oxygen species and other free radicals in normal and disease processes has become a major area of interest in the medical scientific community. In the past 30 years, this area of study has advanced from outright rejection, to general acceptance, to intense study. While there is still some dispute as to the exact role of these highly reactive molecules in pathology, it is clear that they are present in and influence many biological processes. This book provides an overview of the possible biological effects of reactive oxygen species and other free radicals with an emphasis on pathology. The various types of free radicals that may affect the body are discussed along with the potential sources of free radicals, both internal and external to the body. The extensive defenses the body raises against the effects of these molecules in the form of enzymatic and non-enzymatic antioxidants is reviewed. A variety of conditions in which free radicals have been proposed to play a role are discussed. These include the physiological effects of oxygen stress in aging, exercise, and pregnancy. Pathologic conditions discussed include cancer, liver cirrhosis, respiratory problems, and others.