The NATO Advanced Study Institute entitled "Surface Membrane Receptors: Interface Between Cells and Environment" was held in Bellagio, Italy September 13-21, 1975. This meeting was an attempt to bring together in an international and interdisci­ plinary forum scientists who are studying recognitive phenomona which take place at the surface membrane of cells. While an attempt was made to restrict the subject areas covered at the meeting to those experimental systems which have been biochemi­ cally characterized to some extent, it will also be noted that some contributions to this volume represent a preliminary iden­ tification of interesting regulatory substances which might reasonably be expected to act at the cell surface. This book is divided into four sections reflecting the subject areas covered during the course of the meeting. The first section entitled "Membrane Structure and Receptor Function" is intended as an overview of the role of membrane structure in determining the regulatory properties, physical state, structure and location of cell surface receptors. It should be noted that the plasma membrane itself provided the unifying theme for the intention­ ally diverse contributions to this volume. The following three sections represent an arbitrary division into three levels of structural complexity of the things in their external environ­ ment with which cells must specifically interact.