This volume emphasizes the comparative approach to under­ standing vertebrate renal function. I am convinced that this approach is of particular value in understanding both the details of renal function at the cellular and subcellular levels and the renal role in regulating fluid volumes and solute concentrations. My exposure to this approach first occurred during a student research experience in the laboratory of Wilbur H. Sawyer, who also provided an introduction to the works of Homer W. Smith and August Krogh. The importance of this approach was reinforced by doctoral and postdoctoral research in the laboratory of Bodil Schmidt-Nielsen. It has been confirmed through years of personal experience since then. My research and my understanding of renal function have been aided through the years by collaboration and discussion with numerous students and associates. Of particular impor­ tance in developing my views on comparative renal function, and especially on the relationship of structure to function, has been my long association with my colleague and friend, Eldon J. Braun. Donald S. Farner, who suggested the writing of this volume, provided valuable editorial assistance. Much of my personal research in this area has been supported over the years by grants from the United States National Science Foundation and National Institutes of Health. The writing of this volume was completed while I was in Wiirzburg, Federal Republic of Germany, supported by a Senior U.S. Scientist Award from the Alexander von Humboldt Foundation.