Lepton number violating processes and Majorana neutrinos

We discuss some generic properties of lepton number violating (\Lv) processes and their relation to different entries of the Majorana neutrino mass matrix. Present and near future experiments searching for these processes, except the neutrinoless double beta decay, are unable to probe light(eV mass region) and heavy(hundred GeV mass region) neutrinos. On the other hand due to the effect of a resonant enhancement, some of \Lv decays can be very sensitive to the intermediate mass neutrinos with typical masses in hundred MeV region. These neutrinos may appear as admixtures of the three active and an arbitrary number of sterile neutrino species. We analyze the experimental constraints on these massive neutrino states and discuss their possible cosmological and astrophysical implications.