  A weak scattering potential imposed on a $CuO_2$ layer of a cuprate
superconductor modulates the local density of states $N(x,\omega)$. In recently
reported experimental studies scanning-tunneling maps of $N(x,\omega)$ have
been Fourier transformed to obtain a wave-vector power spectrum. Here, for the
case of a weak scattering potential, we discuss the structure of this power
spectrum and its relationship to the quasi-particle spectrum and the structure
factor of the scattering potential. Examples of quasi-particle interferences in
normal metals and $s$- and d-wave superconductors are discussed.
