Terahertz CTHz) radiation has much potential in biomedical and agricultural imaging, security screening and pharmacology applications. One source of THz radiation that is potentially well-suited to applications is the quantum cascade laser (QCL). However, the majority of THz QCL-based systems developed to date rely on incoherent detection, whereas coherent detection could enable much higher detection sensitivities, as well as providing phase information on samples under interrogation. The aim of this work is to progress towards the development of a coherent detection system for continuous wave (CW) THz QCLs employing CW semiconductor laser diodes The proposed scheme aims to take advantage of the compact nature of THz QCL sources coupled with their high optical power, which gives rise to THz field strengths that are 2 to 3 orders of magnitude greater than for continuous wave-photomixed sources. The narrowband nature of THz QCls (about 50 kHz) will also expected to allow a high degree of phase coherence. The free-space electro-optic sampling (FSEOS) detection method is proposed for this scheme. Chapter 2 presents the characterisation of a range of THz QCL sources in order to assess their suitability to operate in CW mode with a relatively high output power which will be used as the THz source of radiation in this coherent system. Then chapter 3 of the thesis discusses the development, implementation and optimisation of an optical phase-Jocked loop scheme for synchronising two CW external cavity diode lasers (ECDLs), which are ultimately intended for use as the probe beam for FSEOS of THz QCLs. Following that, the development of the optical sampling detection scheme employing an electro-optic crystal is presented. When using an electro-optic crystal with high power THz radiation, a second detection mechanism is observed and determined to be from thermo-optic (TO) origin. This is the main contribution in this work, analysis and the developed new model shows an agreement with the measured data for the first time. It is shown that this mechanism enables detection of the THz intensity, rather than the electric field. The thermo-optic response is quantified in terms of THz QCL power and not polarisation and the anisotropic model strongly influenced the magnitude of thermo-optic response. This can be found in chapter 4 and has been published in three conferences. Finally, a system to enable the coherent detection of THz QCLs using FSEOS with CW ECDLs as the probe beam is designed and setup, and initial measurements to observe the coherent interaction between THz QCL radiation and the probe beam are carried out