Lipid peroxidation occurs in physiological conditions and it consists in oxidation of polyunsaturated fatty acids, which are basic biological membrane components. There are two pathway of lipid peroxidation: nonenzymatic and enzymatic. Both of the processes result in generation of various reactive products. Nonenzymatic lipid peroxidation consists in free radical oxidation of polyunsaturated fatty acids while in enzymatic lipid peroxidation the generation of lipid hydroxyperoxides is achieved by insertion of an oxygen molecule at the active centre of lipoxygenase. In this manuscript mechanisms of two processes are described in details. We concentrate on final products of oxidation fatty acids: isoprostanes and neuroprostanes which are regarded as the most important biomarkers of lipid peroxidation as well as on the role of enzymes participating in enzymatic lipid peroxidation process. Moreover we discuss catalytic activity of glutathione peroxidases and transferases--enzymes preventing results of oxidation polyunsaturated fatty acids.