A model of plasma etch anisotropy is presented which shows that the ionic component of the chemical etch process produces etch profiles that vary with , where is the electric field controlling the ion transport to the etch surface and is the etch gas pressure. From a relation derived here between and the observable rf current density, , this model predicts that the profiles will vary continuously from isotropically to anisotropically produced as the parameter is increased. This behavior reflects the directionality of the ion kinetic energy transported to the etch surface and the effect of the ion energy in reducing the surface reaction activation energy. The one‐to‐one correspondence between ion energy and ion transport or etch directionality shown by this model clarifies the compromise required between etch anisotropy and the adverse effects of such high ion energy, such as reduced resist survival and selectivity against other materials in addition to the possibility of increased substrate damage. Another important practical consequence of this work is the establishment of a theoretical basis for extending the conventional wisdom that low pressures (e.g., 1000 Pa) or low pressures from the same ratios. Alternatively, it will be shown that reactive ion etching is a low pressure limiting case of plasma etching, both of which are able to produce identically anisotropic etch profiles, although the latter is capable of considerably higher etch rates. Finally, this model predicts a dependence of the anisotropy on the ion‐neutral scatterer collision cross section and/or mass ratio, which is experimentally confirmed here by previously unpublished data.