  We develop a theory to study apertureless scanning near-field optical
microscopy which takes into account retardation, higher multipoles of the tip,
and the multiple scattering between the tip and the surface. We focus on
metallic systems and discuss the implication of the formation of tip-induced
surface plasmon modes in the tip-surface system. We discuss the effects
associated with the shift in energy of those modes as a function of the
tip-surface distance. Both the local field and the scattering cross section are
enhanced when the tip approaches the surface, but there is no general
correspondence between the two enhancements.
