The current state of the spectroscopy of image states by two-photon ph
otoemission is reviewed. The high resolution permits the detection of
several members of the Rydberg series and the determination of the int
rinsic linewidth of these states. Recent results for metal overlayers
show that the concept of the local work function can be applied and th
at two-photon photoemission is a sensitive probe for the potential at
the surface.