Description:
<jats:p>The relation of neutrino masses to neutrino oscillations and the nuclear double beta decay is highlighted. In particular, the neutrinoless<jats:inline-formula><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M1"><mml:mrow><mml:msup><mml:mi>β</mml:mi><mml:mo>+</mml:mo></mml:msup></mml:mrow></mml:math></jats:inline-formula><jats:inline-formula><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M2"><mml:mrow><mml:msup><mml:mi>β</mml:mi><mml:mo>+</mml:mo></mml:msup></mml:mrow></mml:math></jats:inline-formula>,<jats:inline-formula><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M3"><mml:mrow><mml:msup><mml:mi>β</mml:mi><mml:mo>+</mml:mo></mml:msup></mml:mrow></mml:math></jats:inline-formula>EC, and resonant ECEC decays are investigated using microscopic nuclear models. Transitions to the ground state and excited 0<jats:sup>+</jats:sup>states are analyzed. Systematics of the related nuclear matrix elements are studied and the present status of the resonant ECEC decays is reviewed.</jats:p>