We discuss two recent applications of scanning tunneling spectroscopy of surface electronic states. First, a long standing discrepancy between experimental and theoretical values for the lifetimes of holes in the surface state electron bands on noble metal surfaces is resolved with both found to have been in error. The ability of the scanning tunneling microscope to verify surface quality before taking spectroscopic measurements is exploited to remove the effects of defect scattering on experimental lifetimes. A theoretical treatment of inelastic electron-electron scattering is developed which explicitly includes intra-band transitions within the surface state band. In our model two-dimensional decay channels dominate the electron-electron interactions that contribute to the hole decay, screened by the electron states of the underlying three-dimensional electron system. Second, from a single Mn adsorbate placed within a geometrical array of adatoms on Ag(111) modification of the electronic structure is observed. The changes result from coupling between the adsorbate level and surface electronic states of the substrate. These surface states are scattered coherently within the adatom array, mediating the presence and shape of the array to the adsorbate within. The dimension and geometry of the adatom array thus provide a degree of control over the induced changes.