The recent development in neutrino physics has uncovered finite, but tiny neutrino mass. The most successful theoretical framework to explain this remarkable feature of neutrino is the seesaw mechanism. The idea of the seesaw mechanism was originally proposed in 1979 independently by T. Yanagida and by M. Gell-Mann, P. Ramond and the late R. Slansky.

In order to commemorate 25 years of the seesaw mechanism at one of the places where the idea emerged, we are planning to organize the Fujihara Seminar "NEUTRINO MASS AND SEESAW MECHANISM" to be held at KEK in February, 2004. However, historical recollection is not the main purpose of this seminar. In addition to the Super-Kamiokande, SNO, KamLAND, and K2K experiments, we will expect important new results on neutrino flavor, mass, and mixing from MiniBooNE in a year or so. It will be of great importance to assess critically the seesaw mechanism and its implications on the unification of forces, the origin of the family structure, and the origin of the baryon number of the universe, in the light of new experimental results.