## 6 Conclusions  
The fundamental idea behind 3 MA was born more than three decades ago. The main 3 MA principle, i.e., the multiparametric approach, is based on the combination of measuring information from several micromagnetic methods and several measuring parameters. This allows for avoiding measuring ambiguities and offering the possibility to not only detect qualitative changes, but also to determine the quantitative values of the target quantities. The combined methods differ in their interaction mechanisms and interaction depths, and therefore their combination allows for the influence of disturbances (e.g., temperature, batch variations) to be minimized and different material depths to be sampled at once. The authors are convinced that the method combination is crucial in achieving correct and reliable results in the application of micromagnetic methods. Without this approach, the 3 MA technique would not be so accepted in this dimension in research and industry.  
So far, a broad range of applications on a variety of materials and components has been reported. 3 MA was successfully used to quantitatively determine hardness, hardening depth, residual stress, parameters of static and dynamic tests (tensile, bending, fatigue, creep, impact testing), and microstructure features (texture, cementite, retained austenite). Meanwhile, some applications have been developed to an impressively high degree of technical maturity. Today, specialized 3 MA systems are available for the testing of semi-finished products, such as strip steel and heavy plates, as well as all types of ferromagnetic steel or cast iron components, such as cold- or hot-formed parts, machined (milled, turned, grinded, etc.) parts, and heat-treated (through, induction, case hardened, etc.) parts. Nevertheless, there are still application possibilities to be explored and to be turned into industrial-suited 3 MA systems. Current research activities are concerned with the implementation of 3 MA into closed-loop control schemes. In the future, advanced 3 MA systems will be able to identify individual products. The experimental effort towards 3 MA calibration will be reduced, and the 3 MA hardware and software will be further developed in regard to costs and large-scale manufacturability. Hence, ample work remains, at least for the next three decades.  
[PERSON] and [PERSON] wrote the manuscript and contributed to the methodical development. [PERSON] and [PERSON] developed the optimization and calibration set-ups for 3 MA application. All authors were involved in experimental investigations.  
Part of this research was funded by Bundesministerium fur Bildung und Forschung, (BMBF, former BMFT), Project Number: BMFT RS 150.334, European Research Fund for Coal and Steel (RFCS, former ECSC), Contract Numbers: 7215-PP/055, 7210-PR/248, 7210-PA/339, RFSR-CT-2005-00044 and Forschungsvereinigung Antriebstechnik e.V., FVA-Forschungsvorhaben Nr. 453. The APC was funded by Fraunhofer Gesellschaft.  
The authors would like to respectfully thank the \"inventors\" of 3 MA, who are former colleagues of Fraunhofer IZFP, like [PERSON], [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON]. But special thanks go also to all current colleagues who, for many years, have supported the research and the further development of the 3 MA technique. These include, among others, [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON]. The authors also want to thank [PERSON], [PERSON] and [PERSON] for advice and corrections in writing the manuscript.