The Terahertz gap is the portion of the spectrum lying between 300 GHz and 3 THz. The initial development of Terahertz technology was driven by Space-based instruments for astrophysics, planetary, cometary and Earth science. However, in recent years, the interest of Terahertz science has been rapidly expanded due to the emergence of new applications as secure screening of concealed weapons for military and civil purposes, biological screening, medical imaging, industrial process control and communication technology, to mention some of them. A common characteristic of THz systems is that all of them use quasioptical elements to focus the beams and achieve sufficient signal-to-noise ratios. This doctoral thesis has focus on the analysis and development of quasioptical systems for two different types of THz applications: direct detection for space and heterodyne imaging for security. In the first part, THz absorbers-based detectors for space applications are studied. As this type of detectors can only be studied in reception, their analysis, when located under focusing systems, is usually done by full wave simulations under normal incidence illumination. This method does not describe well the actual coupling to the focusing element when the F/D ratio of the system is relatively small. An spectral model based on Fourier optics has been developed for an accurate and efficient analysis of linear absorbers under THz focusing systems for both small and large F/D ratios. The second part of this thesis is devoted to the optical system of a THz imaging radar for security screening. The goal in this part was to provide an existing THz imaging radar with new capabilities by using quasioptical solutions that do not modify the scanning mechanism and the back-end electronics. On one hand, the radar has been provided with an all-quasioptical waveguide that performs time-delay multiplexing of the beams, reducing the image acquisition time a factor of two by only adding some extra optical elements to the system. Furthermore, the feasibility of this technique to be applied to large linear arrays of transceivers is proven. On the other hand, the radar was provided with refocusing capabilities by implementing the classical optical solution of translating the transceiver.