Lepton-flavour violating processes, such as µ → eγ, are studied in ordinary (non-SUSY) SU(5) and SUSY SU(5) grand unified theories. First given are some introductory argument on the mechanism of U.V. divergence cancellation in flavour changing neutral current processes and on the decoupling of particles with GUT scale masses. We next see that such general argument is confirmed by an explicit ca...