The current results and future perspectives of 2β decay research are reviewed. The present status of neutrino physics makes it necessary to enhance the sensitivity of 2β decay experiments (in terms of the half-life limit for the neutrinoless mode) to the level of ${10}^{26}–{10}^{28}\mathrm{yr}.$ Requirements for future supersensitivity projects are formulated and it is concluded that such a goal will certainly be reached in the most realistic next generation experiments (e.g., CAMEO, CUORE, GEM, GENIUS, and MAJORANA), where restrictions on the neutrino mass may be pushed down to $m_{\nu }<~0.01–0.05\mathrm{eV}.$ In addition, the GEM and GENIUS projects may advance the best current limits on the existence of neutralinos—as dark matter candidates—by three orders of magnitude, and at the same time may be able to identify unambiguously the dark matter signal by detection of its seasonal modulation. All of these results will provide crucial tests of the key theoretical models of modern astroparticle physics and cosmology.

DOI:https://doi.org/10.1103/RevModPhys.74.663