The role of HDL and its main protein component the apolipoprotein Al as being antiatherogenic is well established. Experimental data give support for the involvement of at least three different types of mechanism:(1) the reverse cholesterol transport,(2) anti-inflammatory mechanisms and (3) antithrombotic mechanisms. Depending upon the stage and type of atherosclerosis, different mechanisms may be more or less important. Knowledge of pharmacology of apolipoprotein Al has strongly increased during past years and clinical studies have started with infusions of apolipoprotein Al phospholipid complexes.