Theoretical understanding of the origins of strong Raman signals from molecules near roughened Ag, Cu, and Au surfaces has progressed to the point where the action of two very different types of process are recognized. There is "chemical" enhancement that is apparent only for chemisorbed species, as for example, specifically adsorbed anions on Ag electrodes. The chemical mechanism is poorly understood, however, ingenious ideas and concepts have been advanced, including localization of electron-hole pair states and optically driven charge transfer between adsorbate and metal, to model aspects of the chemical enhancement. The second and best understood process is the enormous enhancement of the incoming and outgoing radiation fields at the surface resulting from lightening rod geometries and the excitation of localized surface plasmon modes. Calculations show that for special geometries, very high (approximately 106-108) enhancement factors are possible in confined regions