This PhD dissertation presents and analyses various room-temperature circuits for Terahertz detection and generation implemented in CMOS 65nm bulk and 28nm FDSOI throughout the course of the thesis. The work discusses the methodology of design and feasibility of fully-integrated focal-plane arrays of detectors in CMOS technologies as potential commercial solutions for various THz applications. The interesting characteristics of the Terahertz portion (300GHz-3THz) of the Electromagnetic spectrum incite plenty of applications ranging from safe and non-invasive medical imaging (cancer detection, dental imaging, pharmaceutical and other), security screening and chemical detection, safety inspection and quality control, astronomy, ultra-high data-rate communications and many others. However, this region of the Electromagnetic spectrum has been dubbed the THz-Gap due to the lack of commercial sources and detectors. Classical THz-systems, therefore, have been explicitly dominated by expensive technologies that suffer from low-integration levels and high operational costs. Consequently, current THz-products have been limited to single or few pixels only with raster-scanning techniques to produce single THz image-frames. Therefore, and contrary to the current state-of-the-art, developing such applications with commercial viability will require portability and high integration-levels, video-rate speeds, low power-consumptions as well as room-temperature operation. Reasonably, Silicon-based technologies that are the core of the vast majority of commercial and high-end electronic products seem to be a tempting solution to bring this THz-Gap. The investigations of this PhD thesis evolve from the theoretical analysis to the optimisation of naked detectors implemented in various technology nodes and illumination topologies, up to the implementation of a 1 k-pixel video imager that includes on-chip signal multiplexing, amplification and processing. Terahertz source design based on 5-push harmonic oscillators is discussed and aimed at attaining the highest frequencies possible in CMOS. Terahertz imaging systems are also discussed in the context of their corresponding applications, link budgets and feasibility.