Though it was quite beyond my original intentions to write a textbook, the book is often used to teach graduate students. To alleviate their misery I decided to extend the introductory chapters and spend more time discussing such topics as the equivalence of quantum mechanics and classical statistical mechanics. A separate chapter about Landau Fermi liquid theory is introduced. I still do not think that the book is fully suitable as a graduate textbook, but if people want to use it this way, I do not object.

Almost 10 years have passed since I began my work on the first edition. The use of field theoretical methods has extended enormously since then, making the task of rewriting the book very difficult. I no longer feel myself capable of presenting a brief course containing the ‘minimal body of knowledge necessary for any theoretician working in the field’. I strongly feel that such a body of knowledge should include not only general ideas, what is usually called ‘physics’, but also techniques, even technical tricks. Without this common background we shall not be able to maintain high standards of our profession and the fragmentation of our community will continue further. However, the best I can do is to include the material I can explain well and to mention briefly the material which I deem worthy of attention. In particular, I decided to include exact solutions and the Bethe ansatz. It was excluded from the first edition as being too esoteric, but now the astonishing new progress in calculations of correlation functions justifies its inclusion in the core text.