Production of tools and component for automotive and energy production includes in most application the adjustment in hardness, toughness, fatigue and wear properties. This is done by heat treatment [4]. Depending on material and Target course different treatment processes are applied like martensitic hardening, bainitic hardening [3], nitriding, nitrocarburizing, carburizing and annealing. Up to now these heat treatment processes are done by recipe.Temperature and time of treatment are measured. No information about the change in material microstructure is available during treatment. So scattering results are the consequence and high security time factors have to be Included to get the necessary process capability. To overcome this difficulty, eddy current sensors, measuring and analysis systems have been developed and are implemented in the hot furnace to measure in-situ the progress of thedeveloping material microstructure during heat treatment of the components.