The authors wish to thank all their friends and colleagues who sent them reprints and preprints, and explained parts of the theory to them. Barry Johnson, Dick Kadison and John Ringrose laid the foundation of the subject and wrote about it clearly. We are indebted to them, and particularly to John Ringrose for the two survey articles [Ri3, Ri6] which made the subject accessible and are still essential reading. Many of the beautiful and deep ideas in Chapters 1, 4 and 6 are due to Erik Christensen on his own or in collaboration with one or both authors. Without his mathematical insight, perseverance and encouragement this book would not have been written. It is a great pleasure to acknowledge our deep debt to him.

The patience and understanding of our wives Patricia and Virginia have been invaluable, and we have received much support and advice from Roger Astley and David Tranah of Cambridge University Press. Our special thanks go to Robin Campbell who typed the entire manuscript with great expertise.

While writing this book, both authors were supported by a NATO collaborative research grant, and the second author by a grant from the National Science Foundation. We take this opportunity to record our sincere gratitude to both agencies.