The mode of fracture in rocks that necessitate the use of diamond drilling techniques is normally brittle. Such rocks, which may be regarded as polycrystalline materials, maintain their uniaxial mode of brittle fracture up to relatively high constraining pressures, that is pressures higher than those normally associated with diamond drilling. Hence, cutting theories based upon ductile shear fracture should not be applied in the context of diamond drilling. The similarity in magnitude of the depths and widths of tracks cut by a diamond, rule out two-dimensional plane-strain failure models. It is noted that the fracture patterns developed below single diamond indenters in homogeneous glass (which may be regarded as a relatively simple material) is very complex. Hence, the fracture patterns and mechanisms in polycrystalline materials are even more complex. The fracture in such polycrystalline materials is highly dependent upon the anisotropy of the physical properties of the constituent grains, e.g. the spatial attitude of cleavage planes and the variation of the frictional coefficient with crystallographic direction. Diamond drilling studies indicate that soluble oils added to the drilling fluid do not increase the penetration rate of the bit. Soluble oils act as lubricants which reduce the wear rate on the bit and thereby maintain a higher average penetration rate. Single diamond cutting experiments and electron micrographs emphasize the complex nature of the "total cutting" process. In addition to the zone of elastically fractured material formed in the wake of the diamond, previously fractured material is also ploughed out ahead of the diamond. Nevertheless, in spite of the extreme complexity of the fracture mechanisms, the design and performance of diamond bits can be synthesised from single diamond cutting experiments.