On the initiative of the NEMO collaboration we organized the first MEDEX (Matrix Elements for Double Beta Decay Experiments) workshop in 1997 (Prague, MEDEX'97), to discuss methods of calculation of nuclear matrix elements in connection with the nuclear double beta decay. In subsequent MDEX meetings (Prague 1999, 2001, 2003, 2007, 2009, 2011, 2013, 2015, 2017) and Corfu 2005, we have extended the scope of the workshop to include other areas where the same sort of techniques are applied, namely: in dark matter detection, rare electroweak decays, and neutrino-nucleus scattering processes. This year (2019) we celebrate the twelfth edition of MEDEX (MEDEX'19) with a program of talks and round the table discussions which cover practically all of the active experimental collaborations in the field, as well as practically all of the theoretical groups which are working actively in the same topics. The …