The aim of this book is to bring together in one volume the current research and thought on the concept of membrane fluidity as a biological phenomenon. The invited articles are intended to review recent develop- ments in the areas of membrane research covered and to summarize the current concepts and theories in those areas. The authors have been given ample opportunity to present their thoughts and speculation on membrane fluidity and related phenomena in a more expanded form than is usually possible in reviews of this type. It is hoped that this approach will have a stimulating effect on research and theoretical development in the biomem- brane field. The chapters in this book are arranged in three sections, the first of which covers physical studies of membrane fluidity and related phenomena on the molecular level. Included are chapters on intermolecular hydrogen bonding between membrane lipids, thermal analysis of membranes, appli- cation of fluorescence and NMR spectrometry to the study of membrane fluidity, and the effect of drugs and other compounds on membrane stability and fluidity. The second section deals with the regulation of membrane fluidity in microorganisms, plants, and higher organisms by factors such as tem- perature, fatty acid chain length, lipid desaturation, and polar head group structure.