The work of this thesis focuses primarily on the quasi-optical analysis of optical systems in the Terahertz region of the spectrum. This included the development of novel simulation techniques based on the principles of mode matching and gaussian beam mode analysis, which are uniquely suited to the simulation of electrically large optical systems at millimeter wavelengths. Additionally, several commercial software packages were used and experimental measurements conducted for the purpose of both validating the newly developed simulation techniques, as well as simulating / measuring specific elements beyond the scope of the in-house software. The main drivers behind this work were a Technology Research Programme of the European Space Agency, the optical payload design of the future proposed Space Infrared-Telescope for Cosmology and Astrophysics mission and work on the Atacama Large Millimeter Array. Once the simulation tools were developed and proven an analysis of these optical systems was conducted. In the case of the Atacama Large Millimeter Array this involved the analysis of the proposed system, whilst for the other projects of the European Space Agency, although an initial optical system was proposed, much work was also required in maturing the design as well as conducting a standard analysis. This was carried out in conjunction with the other groups of the Technology Research Programme, which included the Terahertz optics group of NUI-Maynooth, the Space Research Organisation of the Netherlands, Rutherford Appleton Laboratory Space, and work groups of the universities of Cambridge and Cardiff. The work of this thesis also considered the technical feasibility and design of a future space mission for the purpose of characterising Earth-type exoplanets up to 30 pc distant. This involved the establishment of the science goals and the associated technical requirements, as well as a detailed optical design of the proposed spectropolarimetric payload.