An Antioxidant is an inhibitor of the process of oxidation, even at relatively small concentration and thus have diverse physiological role in the body. It is a substance that protect our cells against free radicals, which may play a role in heart disease, cancer and other diseases. This review is a comprehensive article, which discusses the classification and sources of antioxidants with their biological activities including the mechanisms of actions. In addition, it gives the most trusted common methods to estimate the activities of antioxidants which are chosen from huge number of methods in literature. The article also reviews the quantitative chemical analyses of the most common antioxidants.