This thesis is concerned with the development and analysis of quasi-optical components used in experiments to measure of the CMB in the terahertz and submillimetre wavebands. I have concentrated on corrugated horn structures in the Planck and QUBIC experiments. A precise analysis of the electromagnetic and optical performance of such instruments is required if they are to reach the sensitivity goals required. The analysis of these components was done through computer simulations and experimentation. There were a number of techniques used to model the horns. These techniques were implemented in NUI Maynooth's in-house simulation code 'Scatter' and a commercial software package known as CST. They were used to predict the beam pattern of corrugated horns including the effects of likely manufacturing errors. NUI Maynooth's Vector Network Analyser was used to confirm the results obtained by the simulations. The results in this thesis show that variations in corrugated horn made with the efectroforming technique could result in significant degraded performance but that small gaps between the metal sheets in platelet arrays are acceptable.