Influence of heterogeneities on the initiation of shear zones in the ductile regime The current thesis contains the results from two experimental and one modelling study focused on the topic of ductile strain localization in the presence of material heterogeneities. Localization of strain in the high temperature regime is a well known feature of rock deformation occurring in nature at different scales and in a variety of lithologies. Large scale shear zones at the roots of major crustal fault zones are considered responsible for the activity of plate tectonics on our planet. A large number of mechanisms are suggested to be associated with strain softening and nucleation of localization. Among these the presence of material heterogeneities within homogeneous host rocks is frequently observed in field examples to trigger shear zone development. Despite a number of studies conducted on the topic the mechanisms controlling initiation and evolution of localization are not fully understood yet. We investigated experimentally and by means of numerical modelling phenomenological and microphysical aspects of high ...