Isospin symmetry breaking in double-beta-decay processes

The nuclear structure physics of double-beta-decay transitions is reviewed starting from the consideration of the isospin symmetry breaking in the nuclear many body problem. The difficulties found in the use of the QRPA and related approximations, in dealing with the calculation of nuclear double-beta-decay observables, are understood in terms of the mixing between isospin collective and intrinsic variables and in terms of isospin breaking approximations. (author)