This thesis concerns the development of quasi-optical techniques for long-wavelength imaging, which was conducted through a combination of experimentation and computer simulation. This work was conducted as part of a SFI-funded research program undertaken by the THz Optics group of the Department of Experimental Physics at NUI Maynooth, the aim of which was to extend existing quasi-optical techniques through experimental measurements and the development of simulation tools necessary for efficient design and analysis of long-wavelength optical systems. Description of the upgrading of the 100 GHz test measurement facilities at NUIM and the results obtained from transmission- and reflection-mode active imaging experiments are presented. Numerical simulation of quasi-optical components and systems using scalar wave diffraction techniques was performed. In particular, Gaussian Beam Mode Analysis (GBMA) was applied to the design and analysis of discrete and continuous phase modulating optical multiplexers (phase gratings) for use at 100 GHz. The use of GBMA for iterative phase retrieval was investigated for application to phase grating design. Several phase unwrapping techniques were also investigated in order to simplify manufacture of phase gratings with difficult-to-fabricate profiles. Results of experimental measurements from a number of test gratings are presented and verified using the Maynooth Optical Design and Analysis Laboratory (MODAL) software package. Further improvements to phase grating design are also presented.