The NATO ADVANCED SUMMER INSTITUTE ON THEORETICAL PHYSICS 1981 st was held in Freiburg, Germany from August 31 until September 11th 1981. It was the twelfth in a series of Summer Institutes organized by German Universities. Its main objective was a thorough comparison of structures and methods of two different branches of Theoretical Physics, name ly Elementary Particle Physics and Statistical Mechanics, and the idea was to exhibit the structural similarities, to trace them until their origins, to compare solution and approximation schemes and to report on those new results and methods in...