This book represents the proceedings of a Satellite Symposium of the XIth International Congress of Biochemistry on "Control of Membrane Fluidity" which was held on July 7, 1979 at the Charles H. Best Institute, University of Toronto, Toronto, Canada. The meeting was organized by M. Kates and A. Kuksis and was supported by the International Congress of Biochemistry. The purpose of the meeting was to review recent progress in many different areas of investigation bearing on the role of lipids in the structural and functional property of the cell membrane commonly referred to as fluidity. The aim was to emphasize the factors controlling membrane fluidity as studied in appropriate in vitro and in vivo experiments. The Symposium included invited review papers and short papers offered by discussants. In assem· bling the book no distinction has been made between the two types of presentations, nor has any significance been attached to the chronological order of their presentation in the Symposium. As a result it was possible to provide a much more coherent and continuous presentation than that available at the meeting.