Synthetic resin materials, or “plastics”, offer so many attractive possibilities, particularly in respect of ease of manufacture, that every engineer must be anxious to assess the suitability of these materials to his own special field. The object of the present paper is to review the prospects of the use of plastics in mechanical engineering by their application to stressed parts, and attention is therefore concentrated on the mechanical properties of these materials. The paper touches on unfilled resins and on moulding powders but the main emphasis is laid on reinforced plastics and resin-treated woods, and on their possible use as structural members, gears, and bearings. At the same time, since the value of any material in engineering practice depends not only on its strength and stiffness but also on its general serviceability, some consideration is given to other characteristics, which may affect the behaviour of plastics under service conditions. A certain degree of molecular orientation is necessary to obtain high strength and stiffness in synthetic resins and, although strong artificial fibres such as nylon have been produced, the strong element in reinforced plastics is more often the natural cellulose fibre. The synthetic resin is used in an unoriented form to stabilize the cellulose fibres so that they can carry compressive as well as tensile load and to bond the fibres together so that load may be transmitted from one fibre to another. One convenient form of fibre-filled synthetic resin is that of laminated sheet, and some research has been done on paper-filled materials in this form. It is shown that adequate bonding and stabilization of the fibres demands the avoidance of voids in the resin-bonded board, but that otherwise the strongest board is that containing the least resin. Completely to fill the voids in the paper base without the use of excess resin or very heavy pressure requires close control of the processes of impregnation and pressing; and the minimum amount of resin necessary depends upon the fibre density of the paper. Close attention to these two aspects has resulted in the development of strong boards with low resin content, which can be completely bonded at low pressures. Most synthetic resins are highly resistant to corrosive agents, but many absorb and desorb water in sympathy with changes of humidity of their surroundings. The amount of water sorbed is not serious, nor is the consequent effect on the strength very great, but the slight swelling and contraction of the materials which results from water sorption may cause trouble in certain applications. The synthetic resins at present available for the bonding of cellulose fibres enable fairly effective use to be made of the strength and stiffness of the fibre; but there is still room for considerable improvement, which should result in higher strengths in compression and shear and in greater resistance to buckling. Otherwise the further development of plastics depends principally on improvements in manufacturing processes and intelligent application of those processes to the specific problems of engineering design.