Strain localization in ductile shear zones: This is a crystal plasticity’s world, but it wouldn’t be nothing without a micro-cracking or a sliding grain Localization of ductile strain in rocks results in development of mylonites, which microstructures always display the signatures of several different micro-mechanisms operating at the grain and aggregate scales. Intense crystal plasticity and dynamic recrystallization coexist with micro-cracking, grain boundary migration and sliding, ductile cavitation and failure, diffusive and solution mass transfer along interfaces. Their chronology of activation, their interactions and their respective quantitative roles in the development of the localization process are still unclear. Therefore, a simple inference of the overall mylonitic rheology on the basis of one or another of the latter mechanisms seems illusory. In order to clarify the interplay of mechanisms at the onset of ductile deformation, we performed multi-scale full mechanical field investigations of the ductile deformation of coarse grained synthetic rock salt at room and high temperatures. We applied in situ optical (OM) and scanning electron microscopy (SEM) and X-ray micro-tomography (MCT) during uniaxial compression up to 10 % shortening. Digital surface image and digital volume correlation (DIC and DVC) techniques allowed characterizing and quantifying the multiscale organization of 2D and 3D full strain fields. The same localization patterns are observed in 2 and 3D: the macroscopic shear bands appearing at the sample scale refine into mesoscopic localization bands at the aggregate microstructure scale. The highest resolution investigations clearly demonstrate that the latter result from the concomitant and co-operative interplay between dominant crystal slip plasticity (CSP) and minor but necessary interfacial mechanisms, such as grain boundary sliding (GBS), migration, micro-cracking and cavitation. The quantitative analysis of the interfacial mechanism activity evidence only a modest contribution of less than 10 % to the overall strain, but without the latter development of ductile localization could not occur. The interfacial mechanisms (as GBS) are absolutely necessary to accommodate for the plastic strain incompatibilities among neighboring grains, related to the intrinsic anisotropy of CSP. To conclude, the coupling of CSP and interfacial mechanisms is immediate, right at the onset of ductile deformation. Our major conclusion is that during ductile localization in rocks CSP and GBS act as co-operative mechanisms due to the pronounced plastic anisotropy of minerals. At the beginning CSP appears to be the principal strain mechanism, but GBS happens to be best necessary and supporting one. We further suggest for mylonite development that once grain size reduction and phase mixing had occurred, GBS may become the principal actor, allowing for dominant Newtonian rheology and enhancement of localization.