Molecular oxygen is one of most important substances on the earth. Almost all living organisms utilize oxygen for energy generation and respiration. Oxygen is also inherently dangerous to our existence. It is responsible for the development of most free radicals collectively termed as "reactive oxygen species" (ROS). The production of free radicals is mostly associated with the normal biological processes operative in the living cells. The production of free radicals in cells can happen both accidentally or deliberately. The sources of ROS formation include UV light, ozone inhalation, and breakdown of Purine nucleotide in peroxisome and during inflammatory reactions. Reactive oxygen species (ROS) has the potential to initiate damage to proteins, DNA and other cell biomolecules under pathological conditions. Protein oxidation, which results in functional disruption, is not random but appears to be associated with increased oxidation in specific proteins.