The balance between free radicals and antioxidants is disrupted in many diseases. This disruption may be attributed to a number of factors such as the inability of the cells to produce sufficient amounts of antioxidants, the nutritional deficiency of minerals or vitamins, and the excess production of reactive oxygen species. The purpose of this article was to explore the involvement of free radicals in the pathogenesis of diseases that affect animals including hepatic diseases, parasitic infestation, mastitis, kidney damage, and carcinogenesis.