Progress in understanding the nature of the biological membrane has been very rapid over a broad front, but still pockets of ignorance remain. 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. In turn, the identification of these superfamilies has led to new ways of thinking about membrane processes. Many of these processes can now be discussed in molecular terms, and unexpected relationships between apparently unrelated phenomena are bringing a new unity to the study of biological membranes. 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.