Low density lipoproteins (LDL) are pathophysiologically important be- cause of their central role in the disease atherosclerosis and because atherosclerosis is the leading cause of death in developed countries. Many researchers believe that a more detailed knowledge of the struc- ture, function, and metabolism of LDL may eventually lead to a means to control atherosclerosis. For this reason a fairly large research effort has gone into the investigation of LDL over the past few years. The purpose of this book is to collect and summarize in one place most of the pub- lished information on LDL through 1975. To this end more than 1500 references are cited in the papers that make up this volume. The A, B, C apolipoprotein classification system was adopted for use throughout this work. In addition to the A, B, C, and "D" families of apolipoproteins, apoE is used to designate the "arginine-rich" apolipo- protein. This classification system is used because it is far less cumber- some than other proposed classification schemes for apolipoproteins.