Since 1965 the Nobel Foundation sponsors, through grants from the Bank of Sweden Tercentenary Fund, Sym- posia on subjects which are considered to be of central scientific importance and for which new results of a special interest have been reached. The aim of these Symposia is to bring together, by personal invitation, a limited number of leading scientists from various countries to discuss the current research situation within the field and to define the most urgent problems to be solved. One of the most important fields in modern biome- dical research concerns the structure and function of biological membranes. Research on this subject is very active and important scientific contributions appear at an increasing rate. It was therefore considered highly appropriate to devote Nobel Symposium 34 to the struc- ture of membranes in order to get an expert summary of what is now known in the field. The Symposium was held at Hotel Billingehus in Skovde (about 150 km from Goteborg), Sweden, from June 7 to 11, 1976. In addition to the grant from the Nobel Foundation financial support was received from the No- bel Institute of Chemistry of the Royal Academy of Sciences and from the Science Fund of Wilhelm and Martina Lundgren. The Symposium was attended by some 50 scientists. The papers in this Volume had been distributed in ad- vance to all partiCipants. Therefore only summary pre- sentations needed be given at the Symposium and the main emphasis was put on discussions.