Research presented here focuses on experimental and mesoscopic methodologies to statistically establish the links between operating mechanisms at the dislocation scale and those operating at the polycrystal scale. The underlying assumption of this work is the development of experimental procedures, which allow on the one hand to observe and on the other hand, to characterize crys- talline defects on bulk polycrystalline specimen. Therefore, obtained outcomes can be compared relevantly with micromechanical models. First chapter focuses on emphasizing the importance of the role of microstructures in deformation mechanisms. The example of MAX phases, whose microstructure is very anisotropic, is given. An opening on the influence of strain-path changes on plasticity is also done. Then, second chapter summarizes the contribution of electron microscopy, and in particular Scanning Electron Microscopy (SEM) for the fine characterization of microstructures. Third chapter emphases the physics and the optimization of the SEM technique of Electron Channeling Contrast Imaging (ECCI) in order to follow the evolution of microstructures with deformation. Chapter 4, on the other hand, highlights the potential of ECCI for capturing the evolution of crystalline defects in the vicinity of a single interface locally deformed by nanoinden- tation. Illustration is given with a titanium aluminide alloy and a medium entropy alloy. Finally, some models of crystal plasticity are compared statistically and in-situ with experiences in the case of a β-metastable titanium alloy.