Recent progress in the chemisorption theory for gas-solid systems is reviewed. The CNDO method, a semi-empirical molecular orbital theory taking Coulomb interactions into account. has been applied with success to the system of H, N, C, 0, or F on a graphite basal (0001) surface. The Anderson model in a theory of dilute alloys has been shown to be powerful in understanding the nature of adsorption bond. The model treats only Coulomb interactions on the adatom and introduces parameters in the expression of mixing interaction between electrons on the adatom and in the metal. The role of the electron interactions in forming adsorption bond has been investigated in detail by the method of configuration interactions. More recently, it has been shown that, in analogy to the Heitler-London scheme, the non-orthogonality of the adatom and solid wave functions leads to an antiferromagnetic exchange interaction between the adatom and the solid, and that the interaction might induce covalent adsorption-bond.