ABSTRACT
While string theory purports to unify all the fundamental interactions of physics including gravity, there is currently no string model which is fully viable. Pending the discovery of such a model, it is imperative that one look to the bottom up approach to unification. Grand unification is such an approach wherein one attempts to unify the electro-weak and the strong interactions in a single framwork. In this review we will focus on such an approach, specifically, the approach of supersymmetric and supergravity grand unification[l]. Our aim is to discuss the phenomena of baryon and lepton number conservation and their violation in such theories. The grand
unification approach is expected to be valid up to the scale M q ~ 2 x 1016 GeV but it could be valid even up to the string scale of
(1) Beyond that scale one is in the domain of quantum gravity where the full framework of string unification is needed for consistency.
