Nuclear physics aspects of double beta decay

— Comprehensive description of the phenomenology of the ββ decay is given, with emphasis on the nuclear physics aspects. After a brief review of the neutrino oscillation results and of motivation to test the lepton number conservation, the mechanism of the 0νββ is discussed. Its relation to the lepton flavor violation involving charged leptons and its use as a diagnostic tool of the 0νββ mechanism is described. Next the basic nuclear physics of both ββ-decay modes is presented, and the decay rate formulae derived. The nuclear physics methods used, the nuclear shell model and the quasiparticle random phase approximation, are described next, and the choice of input parameters is discussed in the following section. Finally, the numerical values of the nuclear matrix elements, and their uncertainty, are presented. In the appendix the relation of the search for the neutrino magnetic moment to the Dirac versus Majorana nature of neutrinos is described.