Free radicals biology is very complicated in nature. Free radicals play beneficial as well as deleterious role. Free radicals keep at least one unpair electron in its outer most shell. Due to presence of unpair electron, free radicals are oxidizing in nature and collectively termed as oxidants. The deleterious effects of such oxidants can nullify by antioxidant. The antioxidants scavenge the unstable electrons of free radicals (oxidants). Both antioxidants and oxidants together fulfill the principal of redox reaction. The present book describe the overview of chemistry involve in free radicals biology. It is helpful to readers in understanding how free radicals involve in redox reactions. The book is helpful in understanding the possible types of free radicals and their reactions. The imbalances between oxidants and antioxidants can leads to oxidative stress. The present book also focusing the basics of oxidative stress involve in biology.