Hydrogen adsorption and its behavior on Si surfaces is studied by ion beam techniques in the energy range of MeV-keV. Elastic recoil detection analysis employing MeV ion beams is one of the most reliable experimental techniques for direct determination of absolute hydrogen coverages on Si surfaces. Results of its application to Si(1 0 0) and Si(1 1 1) clean surfaces are described. Important new results of the role of adsorbed hydrogen on the growth process or structures of metallic thin films on Si(1 1 1) surfaces also are described. Characterization of the growth process or structure of the thin films, as well as the characterization of hydrogen, is performed by ion beam techniques. (C) 2002 Elsevier Science B.V. All rights reserved.