Aqueous bases, such as KOH, TMAH, and NH@sub 4@F, are the most important class of industrial silicon etchants. The popularity of these etchants is driven in large part by their extreme anisotropy (i.e. their high face-specificity). Relatively little is known about the chemical origins of etchant anisotropy, though. The problem is simple. On an atomic scale, an anisotropic etchant must be highly defect selective, but the study of surface defect reactivity is notoriously difficult. In this talk, I will discuss two new approaches to studying these reactions. On an atomic scale, I will show how defect reactivity can be quantified using a combination of scanning tunneling microscopy (STM) experiments and atomistic kinetic Monte Carlo simulations. This combination yields very detailed information about site-specific reactivity. To complement these rather time-consuming studies, I will also describe a new technique, which uses microfabricated test patterns, to rapidly assay the reactivity of 180 silicon surfaces simultaneously. We use this technique to perform orientation-resolved chemical kinetics experiments. I will show that the orientation-dependence provides additional insights into chemical reactivity. New phenomena, such as orientation-dependent morphological transitions, will also be described.