Free radicals are byproducts of different physiological processes and have important role in metabolic, signalling and developmental pathways. The environment pollutants, smoking, UV radiations and food can also induce the formation of free radicals or pro-oxidants. The excessive production of free radicals has adverse impact on living organisms and can cause oxidative stress leading to extensive cell damage. Free radical mediated oxidative damage plays important role in pathogenesis of cancer, neurodegenerative, metabolic, reproductive and psychological disorders. Living organisms have developed efficient defense mechanisms to counter ill effects of free radicals by employing different antioxidants.This book introduces fundamental life processes that sustain life, types of free radicals, their generation and their effect on biomolecules and cellular structures. The types of antioxidants and mechanisms used to neutralize the ill effects of oxidants are discussed in detail. Plant based secondary metabolites and phytochemicals with antioxidant and therapeutic potential are discussed. Finally this book explains the role of oxidative stress in pathogenesis of disorders in humans.