Optical second harmonic generation (SHG) is a nonlinear process that converts two photons of frequency w to a single photon of frequency 2w. In the electricdipole approximation, this process requires a noncentrosymmetric medium. For thin films present at the interface between two centrosymmetric media¡Ðsuch as the interface between two liquids, the interface between a liquid and air, or for the surface of a variety of insulators and semiconductors¡Ðonly the molecules which participate in the asymmetry of the interface contribute to the SHG. Surface SGH from molecules which exhibit a large nonlinear optical response at a specific wavelength can be used to measure the relative surface coverage of molecules at the interface as a function of the external electrochemical parameters. The polarization dependence of the surface SHG can be used to estimate the average molecular orientation of chromophores at the liquid/liquid electrochemical interface. The potential dependence of the SHG from the interface provides important information on the position of the adsorbed chromophores with respect to the interface.