Some principles of current distribution within recesses that were developed last century by Carl Wagner are a basis for understanding crevice corrosion that occurs by the recently proposed (1985) so-called IR mechanism, where I is the ionic current flowing out of the crevice or pit, and R is the resistance of the crevice/pit electrolyte. Subsequent experimentation and modeling have shown its applicability for describing the two broad classifications of crevice corrosion: crevice corrosion that occurs immediately, and the delayed form that occurs after an induction period. The mechanism's IR > Deltaphi* criterion, where Deltaphi* is the difference between the applied potential, E-SURF, and the electrodepotential of the active/passive transition, E-A/P, has also been applied in grain boundary corrosion and crack initiation, subsurface corrosion inside pores of sintered metals and alloys, rebar corrosion in concrete, pitting corrosion, lace-like pitting, hydrogen absorption and crack initiation during anodic protection, scaling to other size crevices, and describing the critical crevice corrosion temperature. (C) 2003 The Electrochemical Society.