This volume contains the lectures of the third and fourth Poincaré Seminar, both held in 2003. The third one is devoted to Bose-Einstein Condensation: it covers the physics of superfluid liquid helium as well as the recently discovered atomic Bose-Einstein condensates. Major experimental results are presented, together with relevant theoretical approaches and remaining open questions. The fourth one concentrates on Entropy, giving a comprehensive account of the history and various realizations of this concept, from thermodynamics to black holes, and including theoretical and experimental discussions of the corresponding fluctuations for mesoscopic systems near equilibrium.

The book addresses graduates and researchers in mathematical, experimental and theoretical physics alike, serving both as introduction to the subjects and as guide to the frontiers of experiments and research.