Optical spectra of molecules at surfaces have been measured with high sensitivity by new techniques utilizing optical guided and surface waves. The similarities and differences between these methods and microwave spectroscopy will be outlined, as well as a brief comparison with some of the older conventional optical techniques. Both polarization and spatial information of absorbing molecules can be determined by specific experimental arrangements and by computer analyses of the complex wave equation derived from Maxwell's equations. Results are given on the absorption of a monolayer of a cyanine dye on the surface of an optical waveguide and on a silver surface which can support plasmon surface waves. Through the use of these spectroscopic techniques we have obtained new information about molecular monolayers on surfaces and these results will be presented and discussed. © 1979 American Chemical Society.