The answer to the question which we are frequently asked as to whether there is a "good" cholesterol is most probably negative. There seems to be little doubt about the fact that a high serum level of cholesterol, irrespective of whether it is transported by HDL or LDL is undesirable. However, prospective studies have clearly shown that the risk of atherosclerosis is inversely proportional, at a given serum cholesterol concentration, to the HDL-transported fraction. On the other hand it must be borne in mind that HDL represent a very heterogeneous mixture of lipoprotein classes and families and generalizations should be avoided. Lp(a), for example, which is mostly found in the HDL density class may, even, represent an independent risk factor for atherosclerosis and vascular diseases. Thus, the present paper attempts to review current concepts of HDL structure, chemical and immunochemical composition and metabolism. Furthermore, factors are listed which are known to influence the serum HDL concentration. Finally, the most widely used methods for analysis and determination of HDL, alpha lipoprotein and LpA are listed and mean values obtained in our laboratory are given.