The purposes of this senes were discussed in the preface to Volume I: to present "a range of methods . . . from the physical to the physiological . . . in sufficient detail for the reader to use them in his laboratory" and also to describe "the theoretical backgrounds of the methods and their limita­ tions in membrane biology" so that the reader will be enabled "to evaluate more critically and to understand more fully data obtained by methods foreign to [his] usual experiences. " The chapter by Lee, Birdsall, and Metcalfe with which Volume 2 begins accomplishes these twin goals with a thorough description of the application of nuclear magnetic relaxation measurements to membrane biology together with a lucid and succinct integration of the results of such studies into present concepts of the organi­ zation of membrane lipids. This then permits speculation on the physical basis of membrane permeability. The powerful tool of NMR spectroscopy will have even fuller application with