Double beta decay is a very rare process in nature characterised by changing
the ordering number Z by two units and leaving the mass number A constant.
It can basically occur in two modes, with the emission of two electrons and
two anti-neutrinos or the emission of two electrons only. While the first mode is
expected within the current Standard Model, the neutrinoless double beta decay
of nuclei is not allowed and is of outstanding importance for neutrino physics.
It can only occur if neutrinos are its own antiparticle and if they are massive.
Especially for the first property double beta decay is considered as gold-plated
process. However, due to the known smallness of the neutrino mass, the process
is very rare and requires special low radioactive background environments.
After a general introduction into double beta decay, the lecture focusses on
the current experimental searches and results and their implications for particle
physics. An outlook towards future projects and the involved challenges is given,
including a short discussion on nuclear matrix elements.