This book contains fifteen articles by eminent specialists in the theory of completely integrable systems, bringing together the diverse approaches to classical and quantum integrable systems and covering the principal current research developments. In the first part of the book, which contains seven papers, the emphasis is on the algebro-geometric methods and the tau-functions. Essential use of Riemann surfaces and their theta functions is made in order to construct classes of solutions of integrable systems. The five articles in the second part of the book are mainly based on Hamiltonian methods, illustrating their interplay with the methods of algebraic geometry, the study of Hamiltonian actions, and the role of the bihamiltonian formalism in the theory of soliton equations. The two papers in the third part deal with the theory of two-dimensional lattice models, in particular with the symmetries of the quantum Yang-Baxter equation. In the fourth and final part, the integrability of the hierarchies of Hamiltonian systems and topological field theory are shown to be strongly interrelated. In the overview that introduces the articles, Bennequin surveys the evolution of the subject from Abel to the most recent developments, and analyzes the important contributions of J.-L. Verdier to whose memory the book is dedicated. This book will be a valuable reference for mathematicians and mathematical physicists.