Filiform corrosion occurs of painted metals and cannot be prevented by conventional corrosion inhibitors. In order to better understand the corrosion phenomena, the literature was reviewed and the characteristics exhibited by filiform corrosion on different metals were cataloged. Calculations were performed to select corrosion mechanisms that were compatible with these characteristics. The preferred mechanism, compatible with all the primary characteristics, is that oxygen and water reach the corrosion site by diffusing through the porous filiform tail. Experiments were conducted which confirmed this mechanism. The mass transfer occuring inside the active corrosion cell, and unresolved questions concerning the speed and width of filiform growth are also discussed.