## 2 **Summary**  
Microwave/millimeter wave devices and systems have been developed as core technologies in the fields of communication and measurement. The ever-accelerating progress in devices and circuits, leading to integrated and arrayed sensing systems, and data processing, including computer technology, has contributed to the rapid advancement in diagnostic technology. One of the most important contributions is the improvement of spatial resolution, which is one of the drawbacks of microwave/millimeter wave diagnostics. The importance of visualization (imaging) has also been widely recognized for the rapid and effective elucidation of physical mechanisms. By coordinating these improvements, microwave/millimeter wave diagnostics have been employed in various industrial applications. The purpose of this Special Issue is to introduce recent advancements in microwave/millimeter wave diagnostic technologies and the current status of their applications.  
A total of eight manuscripts are presented in this Special Issue. The manuscripts can be categorized into three subjects, considering the above-mentioned issues: (i) the study of monolithic circuit fabrication and array design for sensing systems, (ii) attempts to improve measurement resolution and accuracy, and (iii) various applications including visualization technology. Summaries of each manuscript are described according to the above-mentioned tasks.  
1. [label  
of the chirp frequency width in the range direction, this system was found to offer good spatial resolution in both two and three dimensions. [PERSON] et al. [5] reported on a metamaterial-based super-oscillatory lens designed to operate at sub-terahertz frequencies. This lens was found to have a focal length of 2.5\(\lambda\) (\(\lambda\): wavelength) and a hot-spot size of 0.67 \(\lambda\) at \(\lambda\) = 3 mm. The lens' characteristics of a long focal depth and sub-diffraction focusing capability were found to be effective for the nondestructive detection of small objects, which is currently needed in industry.
* [PERSON] et al. [6] applied a radar reflectometer to actual non-destructive inspection. The principle of this application involves utilizing the Fabry-Perot effect between two dielectric layers. This system enables one to obtain imaging from inside a building wall, that is, the inspection of tile materials attached to a concrete wall. Quantitative evaluation of the adhesion rate was conducted, and an accuracy of 4.1% was obtained. This result was better than that obtained via the hammering method of a skilled worker. [PERSON] et al. [7] proposed a microwave direction-sensitive sensor for metal crack detection. This sensor adopted two feeding ports to obtain perpendicular polarization currents, thus realizing directional detection. The performance of the sensor was verified via simulations and experiments. The width sensitivities of the two feeding ports were found to be 100 and 63.3 MHz/mm. [PERSON] et al. [8] presented a live-standing wood defect detection model. The authors analyzed the contrast source inversion and used a neural network inversion algorithm to conduct simulation experiments on the detection of internal defects in trees. The results show that the model-driven deep learning network detection accuracy can reach 91.6% for single defects within trees, and can reach 86.3% and 78.3% for heterogeneous double defects and complex multi-media defects, respectively. This Special Issue has already been closed. However, this topic is expected to continue to progress in the future.  
I wish to thank all of the authors and peer reviewers for their valuable contributions to this Special Issue, \"Development and Applications of Microwave/Millimeter Wave Diagnostics in Industry\". I would also like to express my gratitude to all of the staff and people involved in this Special Issue. The author declares no conflict of interest. The authors declare no conflict of interest.