Effective Majorana neutrino masses

A generalisation of the neutrinoless double beta decay process is presented. Neutrinoless double beta decay measures only one out of nine possible effective Majorana neutrino masses in case of three flavours. Limits obtained for all the matrix elements - some of them for the first time - are presented using data from $\mu - e$ conversion, neutrino-nucleon scattering, HERA and rare kaon decays. An outlook towards future possibilities to improve on the bounds is given.