Lipoprotein oxidation is thought to play a pivotal role in the evolution of atherosclerosis. Low-density lipoprotein (LDL) is the main source of oxidized lipid in the arterial wall. Oxidation of LDL alters its properties in a number of ways, making it more atherogenic, but oxidation of other lipoprotein classes may also be important. Common mechanisms are likely to contribute to the oxidation of all lipoprotein classes, with enzyme-mediated oxidation likely to be most important. Antioxidant content, fatty acid composition, particle size and the presence of seeding hydroperoxides also influence oxidative reactions. Larger triglyceride-rich lipoproteins are less likely to enter the arterial wall than LDL, but when oxidized will deliver a greater oxidant load to the arterial wall.