The sixth edition of this book sees for the first time a change in its authorship. John Burrows decided that the time had come for him to retire, and Matthew Barber has now taken his place. John was responsible for seven chapters, and they all demonstrate his special ability to write prose that is clear and easy to read, whether the topic is straightforward, or convoluted and opaque, or somewhere in between. John is a master in explaining the law and in teasing out its finer points, and he also excels in commenting on new developments in ways that are helpful and persuasive. We would like to record our very sincere thanks to John for his excellent work in preparing the preceding five editions. Its quality continues to be reflected in this latest edition.