Two-photon photoelectron spectroscopy permits the study of the time evolution of electronic states at surfaces with femtosecond resolution. Image-potential states at metal surfaces provide examples for various fundamental processes in electron dynamics: exponential decay, dephasing, quantum beats and interband scattering. At semiconductor surfaces the relevant time scales can be of the order of picoseconds. For dangling bond states on Si(100) intraband and interband scattering lead to a delayed build-up and slow decay of the electron population.