The optical response of molecucules adsorbed at or near interfaces are known to be strongly modified relative to those of the free molecules. Surface-enhanced Raman scattering is the most prominent example, however practically all molecular optical properties are affected. In this paper we review the electromagnetic theory of these phenomena with particular emphasis on resonance processes. We discuss lifetimes of excited molecular states, absorption, resonance Raman and fluorescence cross-sections, light scattering and emission yields, energy transfer between adsorbed molecules and photochemical processes. The electromagnetic theory of these phenomena incorporates the surface effect on the local electromagnetic field intensity with the surface-induced radiative and nonradiative decay rates to give working expressions for cross-sections, rates and yields of surface optical processes in terms of the incident beam direction, polarization and frequency, geometry and optical properties of the substrate and its environment and of the optical properties, location and orientation (relative to the substrate) of the adsorbed molecule. Available experimental results are in good qualitative or semiquantitative agreement with the theory. In addition we consider the role of cavity sites in surface-enhanced optical processes. We discuss two models for cavity sites, the conical wedge and enclosures between small particles. The latter are shown to be associated with particularly large enhancements both of the local field intensity and of the surface-induced radiative and nonradiative decay rates. Finally we dwell on the optical properties of small molecular particles which, near the molecular resonance, may give rise to strong local field enhancement provided that the molecules respond coherently to the incident radiation field. We show that the overall response depends on the rate of dephasing processes, which act to drive molecules out of phase with each other. The actual enhancements depend on this rate and on the particle size and shape.