This book contains the lectures given at the Fourth School on Statistical Physics and Cooperative Systems, held in Santiago, Chile, December 12-16, 1994. This School brings together scientists working on subjects related to recent trends in complex systems. Subjects dealt with include dynamical systems, ergodic theory, cellular automata, symbolic and arithmetic dynamics, spatial systems, large deviation theory and neural networks. Audience: This volume will appeal to scientists working in the areas of pure and applied mathematics, nonlinear physics, biology, computer science, electrical engineering and artificial intelligence.