  Scanning tunneling microscopy can provide a probe for the detailed study of
quasiparticle states in high-Tc superconductors. We propose that it can also be
used to acquire specific information about impurity-induced quasiparticle
states and the superconducting order-parameter structure. In particular, the
local density of states is found to be sensitive to impurity-induced resonances
and to the symmetry of the order parameter.
