One of the most challenging and still not satisfactorily solved issues in the design of structural masonry is the determination of the stiffness of shear walls. Determining the correct stiffness is of the utmost importance, for example for the deformation-based design of masonry structures. An assessment of the shear wall stiffness has been made, based on the results obtained from the full-scale testing of medium-size and large unreinforced masonry walls subjected to in-plane static-cyclic loading. This paper discusses the experimental results obtained and addresses the question of the influence of different parameters on the shear stiffness of unreinforced masonry walls. The selected parameters are mechanical (boundary conditions, vertical pre-compression, stiffness degradation), geometrical (aspect ratio, size effect) and constructional (presence of different layers in bed joints). A set of conclusions and recommendations for practical application and future research is also given.