Cell proliferation, cell injury and aging are connected with changes in the cell membrane structure. Apparently these changes activate, in mammalian as well as in plant cells, lipases which liberate polyunsaturated fatty acids (PUFAs). PUFAs are the substrates for lipoxygenases which convert them to corresponding hydroperoxides (LOOHs). Lipoxygenases commit suicide by releasing iron ions. LOOHs react with iron ions to generate radicals. Thus, a nonenzymic lipid peroxidation process (LPO) is induced. It is speculated that the change from enzymic to nonenzymic LPO is connected with the switch from apoptosis to necrosis and that LOOHs produced in enzymic reactions are degraded specifically to signal compounds which induce physiological responses, while nonenzymic reactions seem to induce generation of reactive oxygen species, cell death and age related diseases. Enzymic and nonenzymic LPO processes concern all PUFAs not only arachidonic acid. The main PUFA in mammals is linoleic acid. Since these products serve signalling functions, different degradation paths of linoleic-hydroperoxides are described in detail and the physiological properties of LPO products are discussed in relation to aging and age related diseases.