  Motivated by the Super-Kamiokande atmospheric neutrino data, we discuss
possible textures for Majorana and Dirac neutrino masses within the see-saw
framework. The main purposes of this paper are twofold: first to obtain
intuition from a purely phenomenological analysis, and secondly to explore to
what extent it may be realized in a specific model. We comment initially on the
simplified two-generation case, emphasizing that large mixing is not
incompatible with a large hierarchy of mass eigenvalues. We also emphasize that
renormalization-group effects may amplify neutrino mixing, presenting
semi-analytic expressions for estimating this amplification. Several examples
are then given of three-family neutrino mass textures which may also
accommodate the persistent solar neutrino deficit, with different assumptions
for the neutrino Dirac mass matrices. We comment on a few features of neutrino
mass textures arising in models with a U(1) flavour symmetry. Finally, we
discuss the possible pattern of neutrino masses in a `realistic' flipped SU(5)
model derived from string theory, illustrating how a desirable pattern of
mixing may emerge. Both small- or large-angle MSW solutions are possible,
whilst a hierarchy of neutrino masses appears more natural than
near-degeneracy. This model contains some unanticipated features that may also
be relevant in other models: the neutrino Dirac matrices may not be related
closely to the quark mass matrices, and the heavy Majorana states may include
extra gauge-singlet fields.
