The observation of neutrinoless double beta decay would have far-reaching consequences for particle physics. Most prominently, it would give a hint on the origin of neutrino masses and on the scale at which they are generated. We will discuss the effective description of non-standard mechanisms triggering neutrinoless double beta decay and the implications of their hypothetical observation. The microscopic description of neutrinoless double beta decay including a thorough calculation of relevant nuclear matrix elements and phase-space factors allows for the estimation of the corresponding effective couplings and it can also help to pinpoint the dominant type of contribution. While searching for neutrinoless double beta decay, a significant amount of the two-neutrino double beta decay data has been collected by a number of experiments. Although these events are usually regarded and studied as the background of the neutrinoless version of the process, it will be demonstrated they can be also used to probe physics beyond the Standard Model.
