Studies of the elementary chemical-reaction processes of atoms and molecules on Si surfaces, which have been performed mainly by the authors using high-resolution electron energy loss spectroscopy, are reviewed. The adsorbed states, adsorbed sites and adsorbed structures of atoms and molecules, in particular of H, O2, NH3, N and C2H2, and thermal decomposition processes of these adsorbates are discussed in detail. Recent studies utilizing scanning tunneling microscope and lasers, and trends in Si-gas reaction studies are also reviewed.