Gauge theories and their unification

We present an overview of the subject of grand unification. While there are convincing arguments for the three low-energy interactions to be gauge theories, it is by no means necessary for them to merge into one. Yet, given what we know, it is a most compelling scenario. The fact that it can be done with the known set of particles is an indication that it is not so far-fetched an idea. Of course, experiment will tell. Grand unification means that global symmetries such as baryon number and lepton number cannot be exact and one should expect, at some level, proton decay and neutrino oscillations