Plastic deformation of rocks is accommodated by grain-scale deformation mechanisms. The mechanisms of some minerals, such as quartz and olivine are well understood, but other minerals that are important components of shear zones at depth in the crust, such as phyllosilicates and amphiboles, are less well known. We are using field and microstructural observations to identify active mechanisms during deformation and how they vary with rock type, temperature, and differential stress. By combining these observations with lab-derived flow laws relating stress and strain rate in the minerals, we can constrain the rheology of rocks.