Applications of modulation spectroscopy techniques in electrochemical kinetic studies are reviewed. These methods permit direct in situ detection and identification of reaction intermediates and the measurement of kinetic rate constants approaching the diffusional limit. The results of spectral studies of adsorbed monolayers on metal surfaces are discussed with particular reference to surface roughness effects and photon-assisted charge-transfer transitions between the metal-adatom complex (considered to be a virtual bound state) and the conduction band of the substrate. Recent developments in the theory of the electroreflectance (ER) of metals are presented. The primary features are shown to arise from free-electron effects. At high positive biases, field-modulated interband transitions are enhanced owing to the reduced shielding of surface d orbitals. Surface plasmon generation in metals is also readily detected by ER measurements because of the ease with which interfacial optical parameters can be varied.