Rock engineering has now been practised as a discipline for about 50 years, and many sophisticated methods of analysis for use in rock engineering design have been developed. However, analysis is only a tool for use in engineering design, and rock masses are often so complex that realistic modelling of them, even with sophisticated methods of analysis, is impossible. At the other end of the scale, rock mass classification methods have been developed to quantify the rock mass. These ‘numbers’ are now widely used for assessment of stability of underground and surface excavations in rock, for the determination of rock support requirements, and for the estimation of rock mass strength and deformation properties. In considering these areas, the two concepts - design, and prediction of behaviour - will be dealt with. The discussion will be introduced by a consideration of the characteristics of the rock mass, which will highlight the variability in its properties and hence the uncertainty in the behavioural parameters. Examples of real projects will be used to illustrate the discussion.