  The neutral leptonic sector of the Standard Model presumably consists of
three neutrinos with non-zero Majorana masses with properties further
determined by three mixing angles and three CP-violating phases. We derive the
general renormalization group equations for these physical parameters and apply
them to study the impact of radiative effects on neutrino physics. In
particular, we examine the existing solutions to the solar and atmospheric
neutrino problems, derive conclusions on their theoretical naturalness, and
show how some of the measured neutrino parameters could be determined by purely
radiative effects. For example, the mass splitting and mixing angle suggested
by solar neutrino data could be entirely explained as a radiative effect if the
small angle MSW solution is realized. On the other hand, the mass splitting
required by atmospheric neutrino data is probably determined by unknown physics
at a high energy scale. We also discuss the effect of non-zero CP-violating
phases on radiative corrections.
