Active millimetre-wave imaging systems can be employed in security applications to recognise and identify concealed weapons among clothing. Nevertheless, generated images have some limitations in various aspects. These are primarily related with the high amount of noise present in the system, the poor spatial resolution and the insufficient thermal contrast produced by certain scene components. The main goal of this thesis has been to develop a physical optics modelling of the millimetre-wave image formation (theoretical and computer-based) for the optimization of current millimetre-wave systems. To this etfect, we have implemented a comprehensive millimetre-wave image simulator, which was developed to fulfil the existing gap in the millimetre-wave modelling at the time this system was needed. The major advantage of this innovative system is that it offers low-cost and rapid exploration of the effects of various key parameters of the image formation process. Thus, the optimization is carried out by setting up different im- ager configurations with the simulator and by generating the cor- responding synthetic images for different scenes. The enhanced understanding that these synthetic images provide has enabled the improvement of real millimetre-wave images in terms of thermal contrast or noise. This thesis has contributed in three different areas. First, it provides a theoretical model which includes all the stages of millimetre-wave image formation. Second, it describes the design and implementation of a novel millimetre-wave image simulator, which is capable of generating realistic and mathematically validated images. And third, it reports the exploration of different optimizations that may be carried out within current millimetrewave systems regarding both physical and post-processing aspects.