Medium energy ion beam crystallography, using the effects of channeling and blocking, is a technique sensitive to the geometries of solid surfaces. In this paper, we discuss principles, procedures, possibilities, and problems, illustrated by recent results on the Si (100)–(2×1) and –(1×1)2H and the Si (111)–(7×7) and laser-irradiated–(1×1) surfaces. The prominent role of Monte Carlo computer simulations in the preparation and interpretation of experiments is pointed out.