The etching of silicon in based systems proceeds by a sequential oxidation‐followed‐by‐dissolution process. In those composition regions where the solution is very low in and rich in , the rate‐limiting process is the oxidation step. Consequently, electron concentration, surface orientation, crystal defects, and catalysis by lower oxides of nitrogen play an important role. In those compositions where is in limited supply, dissolution of the formed oxide is the rate‐controlling step and diffusion of the complexing fluoride species is the important factor. Therefore, crystal orientation and conductivity type independence as well as hydrodynamic control are the consequences. In order to meaningfully select an etching composition to solve a specific processing problem, it is necessary to understand this composition‐mechanism interaction. Corollary with the mechanism understanding, sample geometry effects have been followed as a function of solution composition. The solution composition plane has been characterized into various regions where the two basic mechanisms interact and specific procesing utilization is shown. Similar results are shown for the system . In addition, a number of particular etching problems are posed, and solutions offered, that make use of these composition characterizations, and show how one can use their information to solve other practical processing problems.