The reinforcement of materials such as mud and clay by hair, straw and vegetable fibres has been long established in man's history, enabling him to improve his buildings and extend his engineering abilities. With the advent of modern synthetic polymers it was rapidly realised that the addition of fibres, flakes and particulate materials to polymer matrices could improve mechanical properties significantly. Fibres and flakes are the most effective and have enabled several polymers with limited properties to compete with long-established metallic materials, reSUlting in cost, weight and processing economies. This is increasingly apparent in the selection of materials for aerospace and road vehicle applications as well as in a multitude of domestic products. Reinforced plastics, both thermosets and thermoplastics, are used in increasingly harsh environments involving elevated temperatures and aggressive conditions. Fibre reinforcement of thermoplastics dominates, and a pattern of increasing replacement of fibre reinforced thermosets by reinforced thermoplastics is emerging. This trend is encouraged by the development of continuous fibre reinforced grades of the newer high-temperature engineering thermoplastics such as polyether ether ketone. The first part of this book reviews the mechanical properties and theories of short fibre reinforcement. The principal reinforcements are reviewed and a separate chapter is devoted to the uses of natural fibres as reinforcements for thermoplastics. This is an interesting and commercially important area, especially for Third World countries v vi Preface where these fibres are grown but are facing severe competition from synthetic fibres in traditional applications such as ropes and matting.