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We treat for the first time the contributions to the linear and nonlinear optical response of noble metal surfaces from surface electronic transitions involving intrinsic surface states, Rydberg states and surface modified continuum states, and in particular formulate SHG, and 3WM in general, in terms of three-step processes involving three virtual surface electronic transitions. The surface electronic transitions lead to differences in reflection by (111) and (110) surfaces and to an anisotropic reflection by the (110) surfaces. They also lead to differences in the energy and dispersion of the surface plasmons on Ag(111) and Ag(110) and to anisotropic dispersion of the surface plasmons on Ag(110). 3WM at the (110) and (111) surfaces of Cu and Ag is discussed in terms of specific three-step processes at high symmetry points in the surface Brillouin zones. Processes involving transitions between intrinsic surface states at the -Y point of the (110) surface Brillouin zone are shown to lead to resonances in the dependence of χ2y′y′z′ and χ2y′y′z′ the frequency of the input, or output EM waves.