This book provides an in-depth understanding of the nature of surface states and, in particular, their relevance to the physics and chemistry of metallic surfaces. Recent experiments reveal that surface states play a key role in a wide variety of surface phenomena. Individual chapters examine the contribution of surface states to reconstruction, non-adiabatic vibrational damping, nonlinear optical response, tunneling, interaction potentials for scattering and physisorption, as well as surface and thin-film magnetism. Altogether they provide an overview of this rapidly developing field.