Topics in Electroweak and Grand Unified Theories.

In the first part of this work the rate for the two-photon radiative decay mode (the process (nu)(,j) ( --->) (nu)(,i)(gamma)(gamma)) is computed for Dirac and Majorana neutrinos. It is shown that under certain circumstances this process dominates over the one-photon decay mode ((nu)(,j) (--->) (nu)(,i)(gamma)). In the second part the possibility of having Majorana neutrino masses of radiative origin is investigated within SU(5) GUT. The relative magnitudes of such masses is shown to be extremely sensitive to the Higgs sector of the theory. In the third and last part of the work the observability of certain fermion mixing parameters of SU(5) GUT in baryon decay, is discussed.