  A sterile neutrino in addition to the three known neutrinos seems unavoidable
if one wants a simultaneous understanding of the results of the LSND experiment
indicating $\nu_{\mu}-\nu_e$ oscillation together with other evidences for
neutrino oscillations such as those from solar and atmospheric neutrino
deficits. A major theoretical challenge then is to understand why the sterile
neutrino is so light. A simple solution appears to be to assume that it is the
lightest neutrino of a mirror sector of the universe which has identical matter
and gauge content as the standard model. After a brief review of the
phenomenology, a recently constructed realistic grand unified model based on
the gauge group $SO(10)\times SO(10)^\prime$ that embeds the mirror universe
idea is presented. Detailed predictions for known and the sterile neutrino
sector are given and their consequences for cosmology are mentioned.

High Energy Physics - Phenomenology