The book describes distinctive features of the terahertz radiation interaction with various materials, including the Earth's atmosphere, liquids, dielectrics, superconductors, semiconductors, metals and 2D-structures. It discusses all types of terahertz sources, including lasers, photoconductive antennas, semiconductor diode and transistor generators and amplifiers, and considers terahertz band vacuum devices. The text offers a comparative analysis of these devices and their perspectivity in the terahertz band, and describes the basic types of terahertz detectors.Also investigated here are the reasons for the appearance of the so-called "terahertz gap" and the means of increasing working frequency and output power of the sources of terahertz radiation. The book also considers the electromagnetics and electron-optical systems of vacuum terahertz devices and their manufacturing technology.