In physiological condition mammalian cells produce a small amount of reactive oxygen species (ROS) which influence biological processes. At high concentration ROS are the mediator of damage to lipids, protein and nucleic acids. Exposure to free radicals has led organisms to develop series of defense mechanism that can protect against oxidative stress. The defense mechanism is represented by non-enzymatic antioxidants and enzymatic antioxidants that are mainly represented by: superoxide dismutases (SOD), catalase (CAT) and, glutathione peroxidases (GPx), glutathione reductase (GR). This article presents short review about structure, properties and functions of the above enzymes in living cells.