Lipid peroxidation is a free radical initiated chain oxidation of unsaturated lipids. With respect to the ubiquity of unsaturated fatty acids in the cellular membranes, the peroxidative damage has the potential to affect many cellular functions. Some of the products of lipid peroxidation are diffusible and can spread the damage far beyond the site of the original free radical attack. There is an interdependency between reactive oxygen species and lipid peroxidation - reactive oxygen species initiate the reactions of lipid peroxidation and are also produced in these reactions as intermediates. The generation of reactive oxygen species can be triggered either by nonenzymatic mechanisms, in which iron ions play the major role, or by a wide range of enzymatic systems. The primary damaging effect of lipid peroxidation is exerted by the interactions with proteins and DNA. These interactions are then revealed at the subcellular (cellular organelles), cellular, and organ levels. The production of lipid peroxides interferes with the regulation of several metabolic pathways. In this review, particular attention is focused on the interaction of non-specifically formed lipid peroxides with the regulatory factors produced by the controlled oxidation of arachidonic acid (prostaglandins and leukotrienes), the effects on ionic pumps and intracellular calcium metabolism, the participation of lipid peroxidation in the ageing process, and the modulation of hormonal regulations by lipid peroxidation. Lipid peroxidation is induced at the level of the whole organism by various extrinsic factors such as ionizing irradiation, physical activity, diet and fasting, and various drugs. There is increasing awareness of the association between pathologic states and lipid peroxidation. Among the most studied are inflammation, ischaemia-reperfusion injury, and atherogenesis. Lipid peroxidation also plays a dual and complex role in cancer. Organisms have developed an efficient multilevel protective system against lipid peroxidation, but this can be overwhelmed by certain pathologies.