Double beta decays and neutrino masses

The present status and perspectives of double beta decays are briefly reported. Neutrino-less double beta decays(0nubetabeta) have been extensively studied to investigate fundamental properties of neutrinos and weak interactions beyond the standard model(SM). The present limits on the 0nubetabeta half-lives give upper limits of 0.3 similar to 3 eV on Majorana nu masses, depending on the nuclear matrix elements M-0nu. They give stringent limits also on the right-handed currents, the Majoron nu couplings, and others beyond SM. Nuclear structure calculations and experimental evaluations for 60 matrix elements(M-0nu) axe discussed. Recent v oscillation experiments suggest non-zero nu masses in the region of < m(nu) >similar to 0.01-0.1 eV. Research and developments for new generation 0nubetabeta experiments are under progress at several laboratories to study sub eV v masses. MOON (Mo Observatory Of Neutrinos) with Mo-100 is of potential interest for spectroscopic studies of 0nubetabeta decays in the region of 0.03 eV.