  In this talk we show how a natural neutrino mass hierarchy can follow from
the type I see-saw mechanism, and a natural neutrino mass degeneracy from the
type II see-saw mechanism, where the bi-large mixing angles can arise from
either the neutrino or charged lepton sector. We summarize the phenomenological
implications of such natural models, and discuss the model building
applications of the approach, focussing on the $SU(3)\times SO(10)$ model. We
also show that in such type II models the leptogenesis asymmetry parameter
becomes proportional to the neutrino mass scale, in sharp contrast to the type
I case, which leads to an upper bound on the neutrino mass scale, allowing
lighter right-handed neutrinos and hence making leptogenesis more consistent
with the gravitino constraints in supersymmetric models.
