Quantum field theory is a set of ideas and tools that combines three of the major themes of modern physics： the quantum theory， the field concept， and the principle of relativity. Today， most working physicists need to know some quantum field theory， and many others are curious about it. The theory un-derlies modern elementary particle physics， and supplies essential tools to nuclear physics， atomic physics， condensed matter physics， and astrophysics.In addition， quantum field theory has led to new bridges between physics and mathematics.
　　One might think that a subject of such power and widespread application would be complex and difficult. In fact， the central concepts and techniques of quantum field theory are quite simple and intuitive. This is especially true of the many pictorial tools （Feynman diagrams， renormalization group fiows，and spaces of symmetry transformations） that are routinely used by quantum field theorists. Admittedly， these tools take time to learn， and tying the subject together with rigorous proofs can become extremely technical. Nevertheless，we feel that the basic concepts and tools of quantum field theory can be made accessible to all physicists， not just an elite group of experts.