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 $��_��-��_e$ 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.

Invited talk presented at the workshop on "New trends in neutrino physics" held at the Ringberg Castle, Germany, May 24-28, 1998; 10 pages