Plastic deformation is a paradigmatic problem of multiscale materials modeling. Relevant processes range from the atomistic scale where the atomic arrangement of a material are of crucial importance for its deformation properties, up to macroscopic scales where deformation instabilities manifest themselves in the form plastic instabilities. The crucial question how defect and microstructural properties link to the macroscopic constitutive equations of continuum mechanics is still not completely answered. Very often it has been assumed that the transition from discrete defects and microstructural features to continuum mechanics can be accomplished by studying single defect models or by performing simple homogenization procedures. We address this problem by numerical simulations of crystalline and amorphous materials.