Why another book on quantum field theory? Today the student of quantum field theory can choose from among a score of excellent books， several of them quite up-to-date. Another book will be worth while only if it offers something new in content or perspective.
　　As to content， although this book contains a good amount of new material， I suppose the most distinctive thing about it is its generality; I have tried throughout to discuss matters in a context that is as general as possible. This is in part because quantum field theory has found applications far removed from the scene of its old successes， quantum electrodynamics， but even more because I think that this generality will help to keep the important points from being submerged in the technicalities of specific theories. Of course， specific examples are frequently used to illustrate general points， examples that are chosen from contemporary particle physics or nuclear physics as well as from quantum electrodynamics.
　　It is， however， the perspective of this book， rather than its content， that provided my chief motivation in writing it. I aim to present quantum field theory in a manner that will give the reader the clearest possible idea of why this theory takes the form it does， and why in this form it does such a good job of describing the real world.