The quantity of information available about membrane proteins is now too large for any one person to be familiar with anything but a very small part of the primary literature. A series of volumes concentrating on molecular aspects of biological membranes therefore seems timely. The hope is that, when complete, these volumes will provide a convenient introduction to the study of a wide range of membrane functions. Application of the techniques of molecular biology has provided the sequences of a very large number of membrane proteins, and has led to the discovery of superfamilies of membrane proteins of related structure. The classic example of the superfamily is the seven helix receptor superfamily, all related in structure to bacteriorhodpsin, and named after the seven trans-membrane a-helices identified in bacteriorhodpsin. This volume explores the structures and functions of this super family.