The overall strategy of our research is to utilize first principles approaches: Density Functional Theory (DFT) calculations and surface science experiments, to understand surface reaction mechanisms and to design new and improved heterogeneous catalysts. This has led to the demonstration of new bimetallic catalysts for the oxidation of ethylene to ethylene oxide. The key has been the demonstration, utilizing experimental and theoretical tools in concert, of surface oxametallacycle intermediates as the species that control the selectivity of this reaction. This discovery permits the influence of catalyst promoters on selectivity to be probed, and new catalyst formulations to be developed. We have proposed a new mechanism of catalyst promotion for this system by alkalis based on surface ion-dipole interactions. We have predicted, using DFT, that Cu-Ag bimetallic catalysts should offer improved performance over silver alone, and have confirmed this prediction by experiment. This represents one of the first examples of catalyst design for improved selectivity based on first principles approaches.