Experiments have been carried out to assess the rock-cutting ability of thermally stable diamond cutters of different sizes and shapes as individual cutting elements or on various drill bits. In general, penetration is proportional to weight on bit and cutter speed, with variations being dependent on cutter shape, size, and pressure environment. However, there is a marked difference in the penetration of the cutters into the rock at equivalent weight on cutter depending upon whether they are single or are mounted on a bit. It is believed that these large differences are related to the inability of the cuttings to escape from the gap between the bit and the rock face, i.e., poor cleaning. This observation holds promise that by improving the cleaning of the bit, even higher rates of penetration may be attained than the values achieved so far (800 ft/hr in Leuders limestone).