Shear zones are the most ubiquitous features observed in planetary surfaces. They appear as a jagged network of faults at the observable brittle surface of planets and, in geological exposures of deeper rocks, they turn into smoothly braided networks of localized shear displacement leaving centimeter wide bands of “mylonitized”, reduced grain sizes behind. The overall size of the entire shear network rarely exceeds kilometer scale at depth. Although mylonitic shear zones are only visible to the observer, when uplifted and exposed at the surface, they govern the mechanical behavior of the strongest part of the lithosphere below 10–15 km depth. Mylonitic shear zones dissect plates, thus allowing plate tectonics to develop on the Earth. We review the basic multiscale physics underlying mylonitic, ductile shear zone nucleation, growth and longevity and show that grain size reduction is a symptomatic cause but not necessarily the main reason for localization. We also discuss a framework for analytic and numerical modeling including the effects of thermal–mechanical couplings, thermal-elasticity, the influence of water and void-volatile feedback. The physics of ductile shear zones relies on feedback processes that turn a macroscopically homogenously deforming body into a heterogeneously slipping solid medium. Positive feedback can amplify strength heterogeneities by cascading through different scales. We define basic, intrinsic length scales of strength heterogeneity such as those associated with plasticity, grain size, fluid-inclusion and thermal diffusion length scale.For an understanding ductile shear zones we need to consider the energetics of deformation. Shear heating introduces a jerky flow phenomenon potentially accompanied by ductile earthquakes. Additional focusing due to grain size reduction only operates for a narrow parameter range of cooling rates. For the long time scale, deformational energy stored inside the shear zone through plastic dilation or crystallographic- and shape-preferred orientation consumes only a maximum of 10% of energy dissipated in the shear zones but creates structural anisotropy. Shear zones become long-living features with a long-term memory.A special role is attributed to the presence of water in nominally anhydrous minerals. We show that water directly affects the mechanical equation of state and has the potential to synchronize viscous and plastic flow processes at geological time scale. We have shown that fully coupled finite element calculations, using mechanical data from the laboratory, can reproduce the basic mode of deformation of an entire mylonitic shear zone. The next step of modeling lies in benchmarking basic feedback mechanism in field studies and zooming into the braided network of shear zone structure, without losing the large-scale constraint. Numerical methods capable of fulfilling the goal are emerging. These are adaptive wavelet techniques, and hybrid particle–finite element codes, which can be run over a computational GRID across the net.