This book is a collection of critical reviews about a diverse group of virus families with two features in common: the stable repository of genetic information in each virus is RNA, and each virus modifies and appropriates a particular patch of the eukaryotic cell membrane system to complete its structure. The reviews take the reader from the level of virus genome structure and expression through the quaternary interactions between virus-specified elements and cellular components that cooperate to produce virus particles. There are spectacular illustrations in this volume, but it is much more than a picture gallery. Reading widely in this book can be an effective antidote to overspecialization: in these pages, you are likely to learn much about viruses and about cells that you didn't know before; you'll discover illuminating parallels between diverse virus families; you'll come away with a sharpened awareness of important things that are still to be learned. Memphis, Tenn. , Summer 1984 David W. Kingsbury Preface This book was written at the suggestion of Dr. David W. Kingsbury made at a work shop on viruses organized by the Multiple Sclerosis Society in Aspen, Colorado, U. S. A. , three years ago. Originally, we had thought to focus on the morphological aspects of viral assembly. Later, during our discussions on the process of budding of enveloped RNA viruses, it became evident that we should include biochemical data in our review and correlate them with the structural aspects of virus maturation.