A fatty streak, the earliest atherosclerotic lesion consists mostly of lipid-laden macrophages. The mechanism leading to accumulation of cholestrol ester in macrophages in vivo is unknown, but it may result from the uptake of modified low density lipoproteins that enter from blood or are modified within the arterial wall. In this review I discuss the major types of modified low density lipoproteins with a special focus on their chemical nature.