Fast spectrum reactors offer the capability for a better utilization of uranium resources. They are in principle capable to produce (“breed”) but also to transmute (“burn”) fissile materials either by means in a critical or in case of lead alloys even in a sub-critical configuration coupled with an accelerator (ADS).. Associated with the fast spectrum, they require not only a different core design but also many other technological solutions to establish a “safe reactor configuration”. Herein, especially sodium and lead (-alloy) facilitate the option to realize a compact and simple reactor design. While lead in contrast to sodium exhibits a higher neutronic efficiency associated with some often advantageous considered safety coefficients sodium cooled reactors proofed already long term operational experience and material compatibility. Irrespective of the liquid metal reactor concept chosen, liquid metals as coolant formulate new challenges with respect to thermal-hydraulics, material
choice, design and system dynamics. Furthermore, the safety demonstration demands a broad validation/verification and in some cases even a modification of presently availalble physical models
and technological solutions for a reliable monitoring and long term operation. After a short introduction into different kinds of some liquid metal reactor options currently developed within international programs the presentation formulates the major challenges related to the use of liquid metals addressing the thermal hydraulics and adapted instrumentation, some material issues and examples for a reliable component design. Herein, the progress attained in the recent years is shown and issues still open are elaborated. The research conducted in the frame of European R&D programs on sodium fast reactors as well as lead and LBE reactors and ADS impacted significantly their design and their operational safety. For some selected examples the safety behaviour of typical incidents in reactor operation are discussed and open issues are addressed.