The durability of the structural materials at high temperature, under irradiation, under mechanical stress and in the presence of the heat transfer fluid, is one of the challenges in the development of 4th generation nuclear reactors, especially reactors cooled by liquid lead (LFR) or for the development of Accelerator Driven Systems which use liquid lead-bismuth eutectic (LBE). For these reactors, in addition of damage due to irradiation, one of the major damages to structural materials could be due to corrosion by liquid metal. Additionally, under stress (mechanical stress or stress due to temperature fluctuations), structural materials can be susceptible to liquid metal embrittlement (LME) or liquid metal accelerated damage (LMAD), i.e. partial or total loss of ductility in the presence of liquid metal and therefore an earlier fracture. Thus, although tough and ductile metallic alloys are selected, they may become brittle when stressed in liquid metal exhibiting thereby the so-called LME. The objective of the presentation is to summarize the knowledge acquired at UMET over more than 20 years on the mechanical behavior of coated or uncoated metallic alloys (martensitic and austenitic) in the presence of liquid Pb or LBE and then to present the main conclusions and the main issues for future researches. LME sensitivity of the materials has been investigated by mechanical tests (monotonic tests (Small Punch Tests and tensile tests) and Low Cycle Fatigue tests), in temperature, in air and in liquid LBE or/and liquid lead and then the study of fracture surface, cracking and damage by SEM EDX-SEM-EBSD or ToF-SIMS analysis. In general, Body-Centered Cubic studied materials are more sensitive to LME than Face-Centered Cubic (FCC) steels especially at temperature lower than 450°C because of their low ductility while FCC alloys are more susceptible at temperatures higher than 500°C. Note that, strain rate, roughness of the surface, microstructure, temperature, oxygen content in the liquid metal appear as parameters that influences LME sensitivity. Taking into account the mechanisms to explain LME occurrence, challenges to avoid LME of the materials will be presented.