The number and size of projects that involve underground excavation are increasing steadily owing to rapid growth of the urbanisation patterns and the increasing demand for excavation of deeper mineral deposits. Our knowledge of rock breaking process has not been developed sufficiently to cope with the demand. An experimental programme was initiated to study the basic fracturing process of rock by a disc cutter. This study indicates that rock fails by crushing and fracturing processes. The resulting cutting groove is formed primarily by two modes of fracture. Shear-tensile fractures, developed in the vicinity of the cutting wedge, produce small crater-like fragments. Tensile fractures, propagating parallel to the surface of the rock, bifurcate abruptly to the surface and form larger fragments. A major tensile crack develops in the rock as a result of the wedging action of the cutter. The fracture patterns vary substantially with the rock tested. In some cases no sub-surface fracture was observed at all. Assessment of the available theories of wedge penetration against the observed fracture patterns shows that the mechanistic theories of Sikarskie, Dutta, etc., fail to recognise the existence of the tensile fracture mode. In this respect the finite element model appears to be more appropriate to the disc cutting process.