  A small neutrino Majorana mass can arise in the Standard Model as an
effective dimension 5 operator. We calculate the renormalization of this
operator in the minimal Standard Model and in its two-Higgs-doublet and
supersymmetric extensions. Renormalization from the scale of lepton number
violation (e.g., the Planck scale or a GUT scale) to the weak scale decreases
the strength of this operator by an order of magnitude or more if the top quark
and Higgs boson masses are large. Neutrino mixing angles also run with
momentum. We show instances where small mixing at a high scale becomes large at
the weak scale, and vice versa.
