This review describes recent advances that have been made in the separation of HDL subfractions by physicochemical and immunological methods and the relationship between the particles obtained by the different procedures. The metabolic interconversions that occur as a result of cholesteryl ester transfer protein (CETP), lecithin:cholesterol acyltransferase (LCAT) and lipase activities and their role in the formation of mature HDL and in reverse cholesterol transport are discussed. Also considered are the alterations that occur in the HDL subpopulation profile in coronary heart disease and possible mechanisms by which HDL may protect against this condition.