From fundamental considerations of the surface chemistry of Si etching in fluoride solutions as well as pore formation and propagation, we develop a mechanistic understanding of what must be known for rational development of etchants: (1) an acidic fluoride solution because the presence of OH- promotes step flow etching, (2) sufficiently high fluoride concentration compared to the oxidant concentration, (3) the oxidant must be able to inject holes into the Si valance band at a sufficient rate, hence E° > ~0.7 V is required, (4) oxide formation needs to be slow or nonexistent so as to avoid the formation of a uniform oxide and associated electropolishing, (5) reduction of the oxidant must lead to soluble products, (6) film homogeneity is enhanced if the oxidant's half-reaction does not evolve gas, and finally (7) the net etching reaction has to be sufficiently anisotropic to support pore nucleation and propagation.