This book arose from lectures I gave at the Free University Berlin over a number of years. The lectures were intended as two-semester+ introductions to graduate students who intended to start theoretical research work either in many-body physics or in particle physics. Being a general broadly-based introduction, the book requires extension into two directions which have to be subjects of continuing specialized courses. What is needed is • a more detailed account of the exciting developments in nonabelian gauge theories which has taken place over the last decade establishimg quantum chromodynamics as a most probably true theory of strong interactions. • a proper presentation of the field-theoretic explanations of the critical proper- properties of many-body systems near second-order phase transitions.Kleinert H.