This Special Issue highlights the rapid advancements and ongoing challenges in the field of millimeter-wave and terahertz technologies, including sources, devices, systems, and applications. With increasing global interest, these technologies have evolved from lab-based experiments to industrial applications, including non-destructive inspection, security scanning, and medical diagnostics. The articles within this issue cover a wide range of novel developments, such as THz polarimetric and time-of-flight imaging, waveguides for vacuum electronic sources, advancements in gas sensing, and metasurfaces for ultrafast THz modulation. Collectively, these studies provide valuable insights into the future of THz wave applications, positioning this issue as an important resource for researchers and engineers working in the fields of sensing, imaging, and communication.
