See-Saw and Grand Unification

I review the profound connection between the see-saw mechanism for neutrino masses and grand unification. This connections points naturally towards SO(10) grand unified theory. The emphasis will be in the supersymmetric theory, but I also discuss salient features of the ordinary SO(10). Particular attention is paid to the crucial issue of the minimal such theory, i.e. the question of the Higgs sector needed to break SO(10) down to the Minimal Supersymmetric Standard Model or the Standard Model. Some essential features of the see-saw mechanism are clarified, in particular its precise origin at the high scale.