The SIMMER-code family has played an outstanding role in the framework and history of mechanistic safety analyses for liquid metal cooled fast reactors. Its unmatched position is related to the extended simulation range from normal operation conditions, via transients and accidents, core melting and core destruction to in-vessel relocation phenomena and post accident heat removal conditions. SIMMER-III is a two-dimensional (2D), SIMMER-IV a three-dimensional (3D), multi-velocity-field, multi-phase, multi-component, Eulerian, fluid-dynamics code system coupled with a structure model and a space-, time- and energy-dependent transport theory neutron dynamics model. The code has originally been developed for the severe accident simulation of sodium cooled fast reactors but has been applied later for other systems with different neutron spectra, coolants and fuels. Because of the launching of major fast reactor projects in Japan, France and other countries, the current paper again concentrates on the code's initial mission, the sodium cooled fast reactor. Since SIMMER-III is expected to become a standard tool for fast reactor safety analysis with likely application to licensing calculations, the code must be demonstrated to be sufficiently robust and reliable. For this purpose, a systematic assessment and validation program of the code has been conducted in the past and is further going on. The status and the most recent results of these validation efforts are presented.