Filiform corrosion has been observed to occur under a wide range of organic coatings and proceeds with the same general characteristics on steel, aluminum, and magnesium. This article describes the standard tests for filiform corrosion, characteristics and mechanisms of filiform corrosion in steels, as well as coated aluminum and magnesium. It concludes with information on the prevention and mitigation of filiform corrosion accomplished through the adjustment of the environment and changes in the substrate and coating systems.