  General classes of mechanisms for generating small neutrino masses are
surveyed from a top-down (superstring) perspective. In particular, string
constructions have motivated various possibilities involving higher-dimensional
operators, string instantons, and wave function overlaps in large or warped
extra dimensions. These may yield small Dirac masses, Majorana masses via the
Weinberg operator, or Majorana masses from a seesaw mechanism, though the
latter typically differ in detail from the more conventional GUT models.
Possibilities for mixing between light active and sterile neutrinos are
surveyed.
