Can impurity effects help to identify the symmetry of the order parameter of the cuprates?

The effects of non-magnetic impurities on the properties of superconductors with order parameters (OP) of different symmetry are discussed. In particular, we contrast the case of a d_{x²- y²} with various forms of an anisotropic s-wave (ASW) gap. The biggest qualitative difference occurs if the phase of the s-wave OP does not change sign.