During the last several years, it has become increasingly apparent that oxidation of low density lipoproteins (LDL) takes place in vivo as a key event in the development of human atherosclerosis (1,2) and vascular dysfunction (3,4). Thus, many laboratories have focused their interest on the in vitro effects of isolated LDLs on a variety of biological systems, including cultured vascular endothelial or smooth-muscle cells, intact arterial preparations, blood cells, and whole organ systems. LDLs were studied in their native form and after in vitro oxidative modification, which generates an LDL with similar characteristics as LDL isolated from advanced atherosclerotic lesions. Most frequently, ultracentrifugation methods were used to isolate LDL from human plasma, and lipoprotein oxidation was induced by metal ions (Cu²⁺ or Fe²⁺). However, the extent of lipoprotein oxidation encircles a wide range and depends on several variables, and one should be aware of the fact that the results obtained with oxidized LDL described in the literature are achieved with differing preparations, ranging from minimally modified to extensively oxidized LDL. It is therefore the purpose of this chapter to describe methods for the preparation and the oxidative modification of LDL with special attendance on the degree of lipoprotein oxidation, and to explain different ways to analyze the oxidative modification.