"Numerical experiments" on the computer can provide important insights into the variables that appear to be most critical in controlling the overall behaviour of the rock or soil mass. This chapter outlines, by way of specific examples, an “appropriate methodology” of modelling to achieve useful results in rock and soil engineering. All of the examples use either the continuum code Fast Langrangian Analysis of Continua (FLAC) or the two-dimensional discontinuum universal distinct element code and its three-dimensional counterpart distinct element code. FLAC is a two-dimensional continuum code and uses an explicit finite difference solution procedure, in contrast to the implicit finite element procedures frequently used by other investigators. Limit equilibrium methods are probably still the most widely used methods for designing slopes in soil and rock. They are convenient to use and provide ready estimates of a "factor of safety".