The authors are happy and relieved that this book has finally seen the light of day after a six-year gestation. It evolved from lectures given by the first author at Stony Brook and by the second author at Boulder some six years ago. At the time, we decided to combine our two somewhat different points of view about twistor theory into a full-scale book on the subject. We feel that it represents the major developments over the past two decades. A few topics are not covered: For example, the twistor approaches to quantum field theory, the quasilocal mass formula, and the work by Donaldson and others on invariants of four-manifolds (the last two developed while this book was being written).

The manuscript went through several typographical incarnations, via typescripts and an older computer code to the implementation of the complete book in TEX, which was carried out by the staff of Rosenlaui Publishing Services, Inc., under a contract with Cambridge University Press. The authors would like to thank the staff of Rosenlaui and CUP for all of their hard work. In particular, we thank Adrianne Hurndell and Emily Nghiêm of Rosenlaui for their painstaking efforts. The art work and copyediting was done in England by CUP staff, and their fine work is greatly appreciated.