A planet is evolving and loosing heat through convection in its solid interior. On Earth, the signature of this convection is plate tectonics, whereby the surface is composed of a mosaic of plates which are moving around. This pattern of convection is due to the complex rheology of mantle rock. The creation of ductile shear zones is a prerequisite for plate boundary formation. However, the mechanisms governing their formation are still unknown. At shallow depths, brittle deformation dominates, but in deeper parts of the plate, brittle failure is much less likely due to the pressure and temperature conditions. Several mechanisms have been proposed to result in localization, but their importance and mutual feedbacks are not yet fully understood. I will discuss several of the proposed mechanisms and show their impact on plate deformation using results from numerical models.