The behavior of localized corrosion sites has been studied using scanning techniques, potential monitoring and simulated pits. The early stages of pitting were found to depend markedly on the properties of the passive oxide film present prior to pitting. These measurements demonstrated that the stability of pitting depended on the role of the passive oxide in confining the aggressive anolyte, keeping it in contact with the actively dissolving surface and preventing its concentration from falling below a level where repassivation can take place. The concentrations of chloride required to maintain active surfaces were determined using pencil-type artificial pits giving a one-dimensional geometry and coupling diffusional and electrochemical effects. The dissolution kinetics were found to depend on the concentration of chloride in contact wih the dissolving surface. Stainless steel and nickel alloys repassivated at high chloride concentrations whereas iron and nickel in general remained active as the chloride concentration decreased. Potential transients and currents from both the passive surfaces and localized corrosion sites were monitored during the early stages of localized corrosion. The measurements have emphasized the difficulties in determining the early stages of localized corrosion.