  In this talk we discuss a natural explanation of both neutrino mass
hierarchies {\it and} large neutrino mixing angles, as required by the
atmospheric neutrino data, in terms of a single right-handed neutrino giving
the dominant contribution to the 23 block of the light effective neutrino
matrix, and illustrate this mechanism in the framework of models with U(1)
family symmetries. Sub-dominant contributions from other right-handed neutrinos
are required to give small mass splittings appropriate to the MSW solution to
the solar neutrino problem. We present three explicit examples for achieving
the small angle MSW solution in the framework of U(1) family symmetry models
containing three right-handed neutrinos, which can naturally describe all quark
and lepton masses and mixing angles. In this talk we also extend the analysis
to the large angle MSW solution.
