Seesaw 25: Proceedings of the International Conference on the Seesaw Mechanism, Institut Henri Poincare, Paris 10-11 June, 2004

Neutrino physics has undergone spectacular developments in the past 6 years, thanks to the wealthy harvest of experimental esults collected mostly by SuperKamiokande (SK), the Sudbury Neutrino Observatory (SNO) and KamLAND. These have stablished the existence of neutrino flavour transitions, for which the most coherent interpretation relies on neutrino oscillations (or, in the case of solar neutrinos, adiabatic flavour conversion) induced by small but non-vanishing neutrino mass differences. The seesaw mechanism was devised much earlier to generically explain the smallness of possible neutrino masses by appealing to a new high scale. It is then striking that this new scale naturally falls close to the Grand Unification scale if it is to account for the observed oscillations of atmospheric neutrinos (at least for a hierarchical mass spectrum). However, theoretical developments did not await this appealing observation, and there has been an intense activity in this field since the first mechanisms for generating Majorana masses were proposed at the turn of the eighties. The seesaw mechanism inspired a lot of works about its realizations in unified theories or about its numerous implications in particle physics and cosmology, such as the creation of the baryon asymmetry of the universe via leptogenesis or, within supersymmetric extensions, the violation of flavour and of the CP symmetry in the charged lepton sector.