Reactive oxygen species contain reactive oxygen produced by living organisms as a result of cellular metabolism. The defence mechanism that prevents damage caused by reactive oxygen species is represented by antioxidants that can be enzymatic and nonenzymatic. Enzymes are protein molecules of tertiary or quaternary structure that have catalytic ability and specificity for certain chemical reactions. Catalase is enzyme from the group of oxidoreductases, it has a tetrameric structure and is use din various industries and medicine. Oxidative stress is an imbalance between ROS and antioxidants. It is the cause of various diseases and damage of molecules (DNA, lipids, proteins). The causes of oxidative stress, ROS, in normal quantities do not harm organisms, but in high concentrations they lead to various modifications of molecules. Peroxisomes are specialized for carrying out oxidation reactions using molecular oxygen, generating hydrogen peroxide and contain catalase to destroy the excess of ROS. The main function of catalase is antioxidant activity. Catalase acts as an agent in the scavenging og reactive oxygen species and decomposes hydrogen peroxide into the water and oxygen. It hydrolises over the million substrates of the molecules per second. The aim of this work is to point out the presence of oxidative stress, the factors that cause it and, conversely, the non-enzymatic and enzymatic antioxidants that can prevent it.