Abstract To extend our understanding of faulting in the Earth’s crust it will be necessary to describe the various physical processes of faulting in terms of boundary and initial value problems. This is not easy to do. Field evidence indicates that faults form geometrically complex systems and time histories depend on the highly nonlinear processes of fracture and friction. The phenomenon of faulting is reviewed starting with a description of the work of E. M. Anderson who demonstrated that a partial knowledge of the boundary conditions under which faulting could occur allows fault types to be classified. However, many commonly observed features of fault behaviour are unexplained by Anderson’s ideas. These features are described and the various attempts to explain them or reproduce them by modelling are discussed. Seismic studies are briefly covered and it is noted that seismically determined stress drops can also be interpreted to show that earthquake faults have displacement to length ratios close to 10-4. A similar value has also been found from field observation of intersections of earthquake faults with the ground surface. It is also pointed out that faults observed in the field are always significantly more complex than the simple geometrical models of earthquake sources used in seismology and that this deserves greater study.