Modified forms of low density lipoprotein (LDL) are associated with increased atherogenicity. Modified LDL, in comparison with native LDL, demonstrates enhanced cellular uptake by macrophages, foam cell formation and also causes the secretion of cytokines and growth factors from arterial wall cells. Non-enzymatic modifications of LDL (proteoglycans, glycosylation, immune complexes) and enzymatic modifications (lipases, oxygenases) were shown to affect the physicochemical (size, charge) as well as the biological (cellular uptake, secretion) properties of the lipoprotein. Of special interest is the oxidative modification of LDL which was demonstrated to occur in vivo. The mechanism of this process involves cellular lipid peroxidation and requires the binding of LDL to its receptor on macrophages. Some of the modifications can render the LDL more susceptible to other types of modifications (lipid modifications, aggregation, oxidation). As atherosclerosis is a multifactorial disease and since lipases and oxygenases exist in cells of the arterial wall, several forms of modified LDL may exist in vivo. These modifications can occur either in parallel or along different stages of atherogenesis. Inhibition of such LDL modifications may arrest the development of the atherosclerotic lesion.