This paper overviews the principles, theoretical analysis and numerical simulation of STM media and their applications in communication systems. We first present a global perspective on the principles of ST modulation, naturally and artificially created STM media and their applications in communication systems... Next, the interaction of artificial STM media with electromagnetic waves is analyzed, and numerical simulation of electromagnetic wave propagation and scattering from these media is provided. Then, the dispersion diagram of the STM medium is studied for the characterization of subluminal, luminal and superluminal ST modulations. In addition, we investigate the effect of length, modulation velocity, modulation frequency, and equilibrium on wave scattering from the STM medium. The extraordinary interaction of the STM medium with electromagnetic waves leads to unidirectional frequency generation and amplification, which are leveraged for the realization of various enhanced-efficiency broadband microwave components that are integrable with circuit technology, lightweight, tunable, and low cost. These applications include, but are not limited to, magnet-free isolators, magnet-free circulators, transceiver front-ends, and pure frequency mixers. (read more)