Atmospheric and Solar Neutrino Oscillations in \nuMSSM and Beyond

We show how a unified description of the various two-flavor neutrino oscillation solutions, allowed by the atmospheric and solar neutrino experiments, are naturally realized within the framework of $\nu$MSSM (MSSM augmented with the seesaw mechanism) and beyond, especially grand unified theories. A general mechanism for achieving maximal mixing to resolve the atmospheric anomaly is discussed, and applied to the flipped SU(5) model. Except in the case of MSSM and SU(5), a light sterile neutrino is an inevitable consequence of our considerations. The bi-maximal neutrino mixing scenario is one of the options considered. Neutrino hot dark matter can arise in models with maximal $\nu_{\mu}-\nu_s$ oscillations. A ${\cal U}(1)$ flavor symmetry, motivated by the charged fermion mass hierarchies and the magnitudes of the CKM matrix elements, plays a central role.