Many recent developments in our understanding of deformation in the Earth's crust come from geodynamic models which incorporate the geometries, boundary conditions and rock rheologies in the subsurface. Unfortunately, the rheology of rocks deforming under natural conditions is poorly known. Currently there are two approaches to quantify crustal rheology. The first approach extrapolates laboratory-derived flow laws for monomineralic materials over several orders of magnitude and the second approach involves the study of microstructures in rocks. Unfortunately both approaches are only partially successful. A major problem here is that conditions in nature are not directly accessible by laboratory experiments and that there is a lack of samples deformed in natu re of which all the deformation parameters are known. These would form a solid basis for validation of the technique of deriving rheological flow law parameters from microstructures of rocks deformed in nature. This project is centered around mullions, whi ch for the first time provide naturally deformed samples for which the rheological flow law parameters and deformation conditions can be determined. Microstructural analysis will allow quantification of the microstructures in these deformed samples by mean s of a number of state-of-the- art techniques (e.g. EBSD, BSE, microprobe, focussed beam XRD microanalysis). By integrating the results, the projects enables to establish the link between deformation, rheological flow laws and microstructures. Ultimately t he project aims to present improved criteria to deduce deformation mechanisms and paleorheologies from deformed rocks, and provide improved input for geomechanical modellers and other geoscientists who study processes that involve deformation of the Earth' s crust.