Over the last decade the volume Membrane Fusion. edited by Poste and Nicholson, has probably served as one of the major sources of review in­ formation on fusion in membrane systems. Since its publication much new information has been collected. New methods of inducing fusion have been invented or discovered, and new applications for fusion have been found. The need for an up-to-date monograph that covers and in­ tegrates these subjects, reviews established material, and rationalizes and integrates the old and the new is thus obvious. This book is the product of efforts to meet this need. Most of the current work in the field of membrane fusion takes place within the context of intact or modified cells. Hence this book emphasizes the plasma membrane. Each chapter is either a review, a report, or a short historical overview, depending, respectively, on whether the subject is large in scope and has a long history, or the subject is in such an early stage of development that most of what is known is still in the hands of a relatively small number of investigators and is best covered in report form.