The goal of this research is the development and application of theoretical techniques that will provide a molecular level understanding of surface processes, especially reaction mechanisms and energetics. Electronic structures are described by an ab initio embedding formalism that permits an accurate determination of energies and adsorbate structure. An overview of the theoretical method is presented and applications to catalytic and photochemical reactions on nickel and platinum surfaces are discussed. Preliminary studies of ethylene production from ethyl adsorbed on nickel are reported. A second study concerns the photodissociation of methane physisorbed on platinum. Results are reported for a methane molecule interacting with a Pt ring model of the surface. Configuration interaction theory is used to sort out states resulting from electron attachment to methane from lower energy states that correspond to metal excitations. A mechanism is proposed for the photodissociation process.