This volume is dedicated to Prof. Hiroomi Umezawa to honour of his retirement from the Killam Memorial Chair of Theoretical Physics at the University of Alberta. It is intended to summarize the contents of a Conference held at Perugia from May 28 to May 31 1992 aimed at bringing together researchers whose activity has been in close touch with the many topics addressed by Prof. Umezawa in his long scientific career.This book is a collection of invited papers on Field Theory and its many applications to describe collective properties of physical systems. The topics range from Condensed Matter Physics to General Relativity. It contains review papers by leading experts on: Finite Temperature Field Theory, Nonequilibrium Field Theory, Gauge Theories, General Relativity, Nonlinear Equations and Complex Systems.The conference occurred at a time of searching for new ways to use the unifying views of modern field theory to provide explanatory paradigms for a wide variety of phenomena. The book is a timely effort in this direction.The contents of the book will be appreciated by a readership fascinated by both the versatility and the rigorous structure of Quantum Field Theory.