The term risk factor is defined as any clinically measurable characteristic that may be used to predict clinical events. In this chapter, we will focus on lipid-related risk factors associated with clinical events resulting from arteriosclerosis involving the major arteries. The anatomical distribution of the arteries and the function of the organs they feed define the clinical syndromes resulting from arteriosclerosis. Epidemiological and clinical trial data that have been used to define risk factors therefore relate differently to the different arterial systems of the body.