  Ever since the Majorana classic work, the nature of neutrino has been one of
the central questions of the weak interaction physics. If neutrino is its own
antiparticle, the immediate consequence is lepton number violation through the
neutrinoless double beta decay. However, colliders such as the LHC offer a hope
of seeing directly the same phenomenon, and moreover the Majorana nature of new
particles needed to complete the Standard Model. I review here the salient
features of these phenomena, and then turn to grand unification as a way of
probing the scale of the relevant new physics. The minimal supersymmetric
SO(10) theory basically eliminates the LHC physics, but leads naturally to the
large atmospheric mixing angle and furthermore predicts $\theta_{13} \simeq
10^\circ$ without any additional symmetries, in agreement with new T2K and
MINOS data. It also assures that the R-parity is exact and thus leads to the
stable LSP as a natural Dark Matter candidate. The minimal realistic SU(5)
theory, on the other hand, predicts the light fermion triplet, below TeV, as a
source of neutrino Majorana mass.
