The last decade has seen an explosive growth in research on the physics and chemistry of metal surfaces. With the advent of modern spectroscopic methods several aspects of metallic surfaces and their interactions with gases could be explained. The standard of clean surfaces required for basic research were obtained essentially in two ways: (a) By ion-bombardment and proper heat treatment of single crystals, and (b) By depositing thin films on suitable substrates under UHV conditions. Chemisorption studies have concentrated on single crystal surfaces largely because of the inherent simplicity due to their well-defined structure. Numerous publications provide convincing evidence of the insights gained and the results obtained. The number of publications dealing with chemisorption on thin metal films is, however, rather small, although the films are technically more interesting than single crystals. Enormous difficulties faced in reliable characterization of the pure film surfaces, as compared to single-crystals, may be partly responsible for the lack of attention given to such films. The present book aims to fill an important gap in the literature on chemisorption studies. Apart from structural characterization methods, the investigation techniques emphasized here are those used solely for thin films or which permit a direct comparison with single-crystal studies. The literature published to date provides practically no mention of the possibilities available for the analysis of energetically heterogeneous surfaces with modern spectroscopic methods. For this reason, the capability of photoelectron spectroscopy, vibrational spectroscopy, work function measurements and optical analysis for characterizing the pure and gas-covered films is particularly emphasized.