During the past decade, since commercial instruments have become available, secondary−ion mass spectrometry has been increasingly applied as a very sensitive technique for surface characterization. Depth−concentration profiling has emerged as one of the most important applications, owing to the high−depth resolution obtainable. The intrinsic depth resolution of some 10 Å is degraded by uneven erosion, sample mixing owing to the atomic collisions, and at very low concentration, profiling by the sample consumption required. Quantitative analyses are more difficult than with other techniques, but are feasible. The utilization of chemical signal enhancement by oxygen flooding or oxygen bombardment is very helpful.