This paper combines experimental evidence at the mesoscopic and microscopic levels of the physical slip underlying plastic deformation in an aluminium polycrystal in the evaluation of crystal plasticity models. At the mesoscopic level, lattice rotations of individual bulk grains during deformation have been measured using high-energy synchrotron radation (3DXRD). At the microscopic level, deformation induced dislocation structures have been investigated by transmission electron microscopy. The performance of two different versions of the Taylor model taking different approaches to the ambiguity problem is evaluated.