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With the exponential growth of the data traffic in wireless communication sys-tems, terahertz (THz) frequency band is en-visioned as a promising candidate to support ultra- broadband for future beyond fifth gen-eration (5G), bridging the gap between milli-meter wave (mmWave) and optical frequency ranges. The purpose of this paper is to provide a comprehensive literature review on the de-velopment towards THz communications and presents some key technologies faced in THz wireless communication systems. Firstly, de-spite the substantial hardware problems that have to be developed in terms of the THz solid state superheterodyne receiver, high speed THz modulators and THz antennas, the prac-tical THz channel model and the efficient THz beamforming are also described to compensate for the severe path attenuation. Moreover, two different kinds of lab-level THz communica-tion systems are introduced minutely, named a solid state THz communication system and a spatial direct modulation THz communica-tion system, respectively. The solid state THz system converts intermediate frequency (IF) modulated signal to THz frequency while the direct modulation THz system allows the high power THz sources to input for approving the relatively long distance communications. Fi-nally, we discuss several potential application scenarios as well as some vital technical chal-lenges that will be encountered in the future THz communications.