The electronic potential distribution at the surface of a crystal is described. The periodic potential of the interior is modified at the surface by a redistribution of charge, which may be developed in terms of a number of related dipole double layers, space charge, and net surface charge. The image, exchange, and correlation fields, and the field of the net surface charge are presented. The relationship of these effects to the work function of metals and nonmetals, and their role in rectification in metal-semiconductor contacts is mentioned.