The surfaces of ionic crystals are, in general, electrically charged, with a compensating subsurface ionic space charge. Although a wide variety of phenomena are expected to result from the effects of this double layer, the surface charge has been well-characterized only for the silver halides. This paper reviews the origin of the space charge phenomenon, its effects on near-surface properties of ionic crystals (and especially, the photographic process), and the experimental techniques that have been employed in silver halide studies. In particular, some recent work, in which sub-surface radiotracer distributions are measured, have yielded quantitative data on the individual formation enthalpies and entropies of the cation vacancy and the cation interstitial ion.