Double Beta Decay

The neutrinoless double beta decay is a very effective tool to study the properties of neutrino. The detection and study of this process may clarify the following questions concerning of the neutrino physics: i) the neutrino nature (Dirac or Majorana type), ii) absolute neutrino mass scale (a measurement or a limit on m1), iii) the type of neutrino mass hierarchy (normal, inverted or quasidegenerate) and iv) CP violation in the lepton phase. General description of different double beta decay modes (two-neutrino and neutrinoless) will be given. The survey of different experimental methods and obtained results will be presented. Prospects for investigation of double beta decay processes in future experiments will be also discussed.