The use of quasi-optical techniques for signal processing at millimetre wave frequencies and their application to spread spectrum systems is presented in this thesis. Millimetre waves offer a number of advantages when compared to microwave and optical signals and as a result they are finding an increasing number of applications in the area of communications. Traditionally, millimetre wave research has been centred on scientific and experimental interests. In recent years, however, practical communications systems have been emerging at these frequencies. New technologies are being developed which aim to exploit the potential advantages of millimetric signals and the work undertaken here relates to one particular avenue - that of quasi-optics. This thesis introduces measurement methods for assessing millimetric components and quasi-optical systems, followed by an appraisal of Gunn oscillators and their associated power supplies as suitable signal sources for communications. A type of Gunn oscillator displaying semi-chaotic behaviour is described which may have potential as an inherently broadband source suitable for spread spectrum applications. The application of quasi-optical signal processing methods to a prototype spread spectrum millimetre wave system is presented. The system has been shown to operate satisfactorily in a number of demonstrations. Theoretical models of the quasi-optical circuit show good agreement with experiment and an analysis of the signal-to-noise behaviour predicts the potential receiver performance. Some ideas for future work, building on that presented here, are suggested.