Double Beta Decay

Double beta decay is a key process to reveal a fundamental property of neutrinos. If neutrinos are Majorana particles, that is they are equivalent to their antiparticles, neutrinoless double beta ([Formula: see text]) decay, [Formula: see text], would occur. The process is beyond the standard model and would lead to a scenario which can explain the extremely small masses of neutrinos and provide a solution to the current matter dominance of the world. In this talk experimental efforts searching for [Formula: see text] decays are presented. Then, major [Formula: see text] experiments together with searches using [Formula: see text]Xe nuclei are described, followed by the current status of the KamLAND-Zen experiment.