Terahertz sources are devices that generate terahertz radiation, which falls between the microwave and infrared regions of the electromagnetic spectrum. Terahertz radiation is of interest for a variety of applications, including spectroscopy, imaging, and communications. Research in terahertz sources focuses on developing new and improved methods for generating terahertz radiation. Common terahertz sources include quantum cascade lasers, photomixers, and gyrotrons. These sources can operate in both pulsed and continuous-wave modes, and can produce terahertz radiation with high power and wide frequency tunability. Overall, the goal of research in terahertz sources is to advance the technology and capabilities of terahertz radiation for various applications, such as medical imaging, security screening, and wireless communication.