Diamonds have many extreme physical properties that lead to a host of technological applications. Their main use as a tool or abrasive for the machining of hard materials involves diamonds rubbing against the material being worked and often against each other. The strength and frictional properties of diamond are thus of great practical, as well as academic interest. Studies of the reciprocating sliding of one diamond on another for extended periods have shown that the changes in friction and wear are linked with the formation of debris. Analysis of the debris shows it does not consist of diamond fragments. Investigations of the effect of low-pressure gases on diamond friction also indicate debris plays an important role, and that the friction and wear are markedly dependent on the state of adsorbants on the diamond surface. Details are also given of the friction and wear of steel, glass and sintered diamond in contact with diamond. An associated study of the solid particle erosion of diamond has shown that cracking can occur at impact velocities of 25 m s<SUP>-1</SUP>. To facilitate the friction studies two existing friction apparatus have been automated. Computer control now allows data to be collected and analysed more rapidly and thoroughly than was possible before. The equipment has also been adapted to extend the conditions under which friction studies can be made.