Overstressing of the rock surrounding a tunnel can result in either brittle fracture of the intact rock or shear failure along pre-existing discontinuities such as joints or shear zones. These two types of failure can co-exist and the extent to which the failures propagate depends upon the characteristics of the rock mass, the magnitude and directions of the in situ stresses, the shape of the tunnel and the intensity and orientation of the discontinuities. Numerical analysis plays an increasingly important role in the assessment of tunnel stability and design of reinforcement and support. Rapid evolution of computer software and hardware offers the potential for the calibration or possible elimination of some of the empirical techniques upon which tunnel designers have to rely.