Scuffing is a severe form of surface damage which limits the performance of lubricated sliding machine components. Empirical work has shown that failure either occurs under relatively mild elastohydrodynamic conditions with barely modified surfaces or under severe conditions with the surfaces well run-in. Two hypotheses exist which may explain these experimental differences. This thesis examines their relevance. The first hypothesis is that, under elastohydrodynamic lubrication, the surface asperities either remain rigid or become elastically deformed-micro-elastohydrodynamic lubrication. A non-dimensional plot, developed by Baglin, predicts the occurrence of the regimes. An experimental study of running-in and scuffing for tests initially operating in the different regimes is described. Tests were run on a two disc machine with incremental loading. Running-in occurred both when tests started in the micro-ehl regime and when they apparently entered it during operation. High sliding prevented entry into micro-ehl; scuffing occurred with barely modified surfaces. This hypothesis discriminates between failure types but cannot alone predict scuffing. The second hypothesis, by Crook and Shotter, is that scuffing represents an inbalance between the rate of film thinning with increasing load and the rate of running-in. Increasing load increases the temperature which, due to its effect on viscosity, controls film thinning. Knowledge of the machine's thermal behaviour is required. A model is developed to predict temperature in a finite length cylinder subject to a discrete rotating heat source and convective cooling. Steps to apply the theory to a two disc machine are detailed and the results compared to previous experimental temperatures. Methods of changing thermal response are considered and preliminary tests with the discs insulated to increase the temperature rise are described. A marked reduction in scuffing load emphasises the importance of thermal design. Further experimentation is necessary to determine whether the Crook and Shotter hypothesis can quantify scuffing failure.