For many years, a huge effort in theoretical surface science has been directed towards understanding the processes of making and breaking bonds at surfaces. The aim is not only to be able to predict reaction mechanisms and pathways, but also to understand how to manipulate these pathways to open up new avenues for the control of surface chemical reactions. Most of the current theoretical work in surface reactivity uses ab initio electronic structure calculations based on density functional theory (DFT). Underlying these calculations is the Born-Oppenheimer approximation, in which the electrons always remain in their ground state configuration. However, recent experimental results have clearly demonstrated the significant role of electronic excitations in a wide variety of surface reactions. Standard DFT approaches are clearly inadequate here, but there has, to date, been remarkably little progress in developing methods that go beyond the Born-Oppenheimer approximation. It is our aim in this proposal to establish a theoretical framework that will provide a quantitative understanding of the effects of electronic excitations in surface reactions.