This paper discusses the basic principles of multi-frequency eddy current testing and explains how it can be used on high-volume production lines to detect faulty heat-treated parts based on case depth, hardness patterns, tensile strength, carbon content, soft spots, and surface decarburization. It also presents examples showing how the method is used in high-speed inspection of cam shafts, screws, balls of various sizes and materials, distance pins, and complex bolts.