"After being frequently urged to write upon this subject, and as often declining to do it, from apprehension of my own inability, I am at length compelled to take up the pen, however unqualified I may still feel myself for the task. " William Withering, M. D. ' I have yet to find a description or a quote that better summanzes my initial ambivalence towards embarking on such an endeavor as partici pating in putting together this monograph. The impetus for The Red-Cell has been a simple, genuine Membrane: A Model for Solute Transport desire to bring together an authoritative account of the' 'state of the art and knowledge" in the red-ceIl-membrane transport field. In particular, it seems important to emphasize the pivotal role the red cell has played for several decades in the discovery and the elucidation of mechanisms of plasma-membrane transport processes. It is only with such knowledge that we can hope to push ahead and make progress in this exciting, multifaceted area. Eventually, one hopes to not only further our knowledge of red cells, but apply the newly gained insights to any other of the plasma membrane. cell with the common denominator In this compendium of reviews, the reader will find that the term model will take on a variety of gists and meanings. In some chapters, the red cell has been chosen as a model membrane solely on the basis of its preeminent design and simplicity.