Application of the techniques of molecular biology to the study of cholesterol and lipoprotein metabolism in recent years has led to a dramatic increase in our knowledge of this field. This brief review provides a general outline of normal lipoprotein and cholesterol metabolism; some recent advances in research are discussed, and the molecular defects present in hypobetalipoproteinaemia, familial apo B-100 deficiency, type III hyperlipoproteinaemia, and familial hypercholesterolaemia are described and their clinical relevance is discussed. New aspects are presented of the finely tuned regulation of cholesterol synthesis, and its importance in treatment of hypercholesterolaemia is discussed, as is its possible involvement in oncogen expression.