The primary concern of this thesis is the analysis of long wavelength quasioptical receiver systems operating within the Terahertz and submillimetre wavebands. Specific attention is paid to the front-end coupling optics of the Band 5 and Band 9 receiver channels of the Atacama Large Millimetre Array (ALMA). The theory of Gaussian Beam Mode Analysis (GBMA) is expanded and developed as the basic analytical tool for the work presented. This technique is utilised to model both classical optics diffraction and interference patterns. An alternate method of describing these diffraction patterns is developed using the Angular Spectrum of Plane Waves (ASPW). The general GBMA technique is supplemented by the commercially available Physical Optics (PO) package GRASP9 developed by TICRA. A comprehensive analysis of the ALMA Band 5 front end optics was conducted in conjunction with the Group for Advanced Receiver Development (GARD). This analysis was developed to investigate the efficiency of various configurations of the optics and was supplemented by a rigorous measurement campaign at GARD. As part of our ongoing collaboration with the Space Research Organisation of the Netherlands (SRON) a series of theoretical and experimental analyses were performed with the aim of improving the cross polar efficiency of the ALMA Band 9 receiver. In addition to these investigations of long wavelength receiver optics the theory of GBMA was combined with mode-matching theory to describe the behaviour of standing waves in typical submillimetre receiver systems. The reflection and transmission amplitude response patterns of several standing wave cavities were predicted with this technique and compared with experimental measurements. The effect of minor alterations to the feed horn structures on the resonance profiles are studied in depth. The eigenmodes of these resonant cavities are also analysed. A complex stray light baffle structure is introduced within the cavity and its effect upon the system is quantified.