  The discovery of neutrino masses has provided strong hints in favor of the
possibility that B-L symmetry is an intimate feature of physics beyond the
standard model. I discuss how important information about this symmetry as well
as other scenarios for TeV scale new physics can be obtained from the baryon
number violating process, neutron-anti-neutron oscillation. This article
presents an overview of different aspects of neutron-anti-neutron oscillation
and is divided into the following parts : (i) the phenomenon; (ii) the physics,
(iii) plausible models and (iv) applications to cosmology. In particular, it is
argued how the discovery of $n-\bar{n}$ oscillation can significantly affect
our thinking about simple grand unified theory paradigms for physics beyond the
standard model, elucidate the nature of forces behind neutrino mass and provide
a new microphysical view of the origin of matter in the universe.
