Quantum decoherence : Poincaré Seminar 2005 Decoherence and the Transition from Quantum to Classical — Revisited Monitoring the Decoherence of Mesoscopic Quantum Superpositions in a Cavity Approaches to Quantum Error Correction Decoherence of a Quantum Bit Circuit Roots and Fruits of Decoherence Dynamical Consequences of Strong Entanglement. 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. .