The key role of the shear strength of discontinuities and rock joints in the behavior of rock masses and in the safety of rock engineering projects where stresses are low when compared with the rock intact, such as dam foundations, slopes and surface excavations, or underground tunnels and caverns, is for long acknowledged. Several failure criteria – namely, Coulomb, Patton, Barton and Grasselli – can be used to derive design parameters from results of field investigations, mainly laboratory tests. The keynote will address some particular issues related to the shear behaviour of rock joints that can be found along the path between field investigations and design parameters: in situ versus lab tests and scale effects, joint surface roughness and its influence on the stress distribution along the joints, roughness wear and repeated shear tests on the same joint sample.