In this talk I will review the motivations for "grand unification" of forces and of matter particles and the experimental hints in its support. These theories naturally predict small neutrino masses, leading to the phenomenon of neutrino flavor oscillations. Such oscillations, which transmute one flavor of neutrino to another, require tiny neutrino masses. Predictions of unified theories for neutrino oscillation
parameters will be discussed and shown to be in excellent agreement with recent data. The hallmark of these theories is the prediction that protons
should ultimately decay, with a lifetime potentially within reach of ongoing experiments. Cosmological connections between proton decay
and the origin of matter in the universe will be discussed.