Ion accelerators have been used by material scientists for decades to investigate radiation damage formation in nuclear materials and thus to emulate neutron-induced changes. The versatility of conditions in terms of particle energy, dose rate, fluence is a key asset of ion beams allowing for well-controlled analytical studies. In addition, very short irradiation times and handling of non-radioactive samples dramatically curtail the global cost and duration as compared to in-reactor testing. Coupling of two or more beams, use of heated/cooled sample holders, and implementation of in situ characterization and microscopy pave the way to real time observation of microstructural and property evolution in various extreme radiation conditions more closely mimicking the nuclear environments. For these reasons, multiple ion beam facilities have been commissioned worldwide.In France, under the auspices of the Universite Paris-Saclay, the JANNuS platform for Joint Accelerators for Nanosciences and Nuclear Simulation comprises five ion implanter and accelerators with complementary performances. At CSNSM Orsay, a 200 kV Transmission Electron Microscope is coupled to an accelerator and an implanter for in situ observation of microstructure modifications induced by ion beams. At CEA Saclay, the unique triple beam facility in Europe allows the simultaneous irradiation with ions (Fe, Xe, W) for nuclear recoil damage and implantation of a large array of ions including gasses for well-controlled modelling-oriented experiments.Examples are given to illustrate the use of JANNuS ion beams in investigating the radiation resistance of structural materials for Generation IV reactors as well as GenII/III and fusion reactors. Materials of interest are ODS steels, model ferritic alloys, austenitic steels and metallic coatings. The main focus of this presentation is on studies pairing in situ microscopy at low to intermediate doses to elucidate the initial steps of damage formation and irradiation at JANNuS Saclay to characterize degradation up to high doses.