The Poincaré Seminar is held twice a year at the Institute Henri Poincaré in Paris. The goal of this seminar is to provide up-to-date information about general topics of great interest in physics. Both the theoretical and experimental results are covered, with some historical background. Particular care is devoted to the pedagogical nature of the presentation. This volume is devoted to Quantum Decoherence. A broad perspective on the subject is provided by the contributions of W. H. Zurek, H. D. Zeh and E. Joos, together with clean up-to-date presentations of the actual experiments on decoherence both in the mesoscopic systems of atomic physics, by J.M. Raimond and S. Haroche, and in the "quantronic" or condensed matter context, by D. Esteve et al. Further, the question of quantum codes and error corrections is discussed in the contribution of J. Kempe. .