Apart from known "visible" sector of theory, the hidden sector probably exists which is composed of singlets (fermions and bosons) of the standard model symmetry group. The hidden sector may have its own symmetries and certain structure, and the neutrinos, being neutral, provide a portal to this sector. Furthermore, the smallness of neutrino mass as well as striking difference of mixing patterns of quarks and leptons can originate from the hidden sector. Arguments in favor of such a scenario embedded into Grand Unification will be discussed. Its simple realizations in the context of SO(10) GUT will be presented. Other manifestations of the hidden sector can include light sterile neutrinos and dark matter.