This volume contains 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 is devoted to Entropy, giving a
comprehensive account of the history and various realizations of this
concept, from thermodynamics to black holes, and includes theoretical
and experimental discussions of the corresponding fluctuations for
mesoscopic systems near equilibrium.