Resin Transfer Moulding (RTM), at present, is a semi-automated, low volume production process for fibre reinforced plastics with much work being undertaken to achieve full automation and medium volume production. The efficient and consistent impregnation of thermosetting resin into a reinforcement pack are key requirements in the development of R TM. For mass production and automation accurate predictions of mould fill times are critical in order to plan production. At present, the fill times are predicted by assuming Darcy's law and using experimentally determined permeability value. Permeability measurements display a large amount of scatter which affects the accuracy of simulations of mould filling. Commercial fabrics such as 'Injectex' are now available which achieve high permeability to resin flow through a modified microstructural architecture of fibre tows. This thesis seeks to examine the relationship between microstructure and permeability in these fabrics in the context of possible decreases in mechanical properties which may result from non-uniform fibre distribution. An image analysis technique is used to characterise and quantify the regions of flow within the fabric architecture which are then correlated with a series of permeability measurements determined experimentally. This then leads onto a mathematical model for the prediction of the permeability of the fibre pack.