Neutrino Masses from the Top Down

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 constructions may yield small Dirac masses, Majorana masses via the Weinberg operator, or Majorana masses from a seesaw mechanism, although the last typically differ in detail from the more conventional Grand Unified Theory models. Possibilities for mixing between light, active neutrinos and sterile neutrinos are surveyed.