It is now possible to write every reaction and name every intermediate in the biosynthesis of cholesterol from acetate with a reasonable degree of certainty. Many of the enzymes involved have been obtained in at least a partially purified form and their properties determined. With respect to the biosynthesis of the other terpenoids, there are still points of detail to be settled, but the broad picture has been outlined. Attention is now being devoted to the more intimate details of the mechanisms involved, to purification and properties of the responsible enzymes, and to factors influencing biosynthesis of isoprenoid compounds.