This book covers the active devices, circuits and systems including antennas used in the electromagnetic spectrum for wireless technology in the GHz to THz frequency range. It also includes the whole domain of digital communication techniques and its use in modern electronic communication including those in 5G and the forthcoming 6G mobile communication. The presentation of the book is in a concise yet complete way and stresses the physical and technical aspects with application view-points but using minimum possible mathematics. The book: Discusses the intricate differences and evolution of GHz (i.e. microwaves) to THz devices and circuits including antennas, useful for wireless technology in this higher frequency domain of electromagnetic spectrum, comparing those with their counter parts at lower RF frequency of KHz to MHz. Briefly discusses various wireless systems in the GHz frequency range—both analogue and digital; like RDAR and radio-aids to navigation, satellite communication, mobile communication, GPS and so forth. Highlights the need and usefulness of digital communication and covers various digital modulation techniques like PCM, ASK, FSK, PSK, QAM and also various analogue and digital multiplexing techniques like FDM, TDM, CDM, OFDM, and NOMA useful for modern digital communication. Discusses the new and emerging technology of wireless technology at THz frequency covering signal generation, waveguide and quasi-optic components including sensors and detectors, antennas, channel modelling, T-ray imaging and so forth. Covers the applications of THz for various emerging new applications like concealed weapon detection (CWD), standoff detection of explosives and abusive drugs, medical application of THz including the use of THz in forthcoming 6G mobile communication etc. This overall scenario of developments of wireless technology from GHz to THz frequency domain is expected to attract the interest of students, research scholars, professionals, and even the general readers alike with a new vision to have a quick yet in-depth glimpse of the progress of wireless technology in the modern times. It is primarily written for senior undergraduate students, post-graduate students, and academic researchers in the fields of electrical engineering, electronics and communication engineering, wireless communication, telecommunication, and computer engineering.