The simulation method is able to relieve the engineer of experimental work during the planning of extrusion plant and thus clearly reduces the development time. Sufficiently precise determination of the calculable parameters is possible with the process models developed so far. The empirical knowledge of the engineer is, of course, necessary as well in order to estimate the impact of influencing parameters that are not taken into account in the calculation models. The presented simulation program REX can perform these complex calculations in a comprehensive manner, with its simple and comfortable user interface. The program, however, can be employed as a valuable tool only if it is in the hands of an expert operator who can, and indeed must, be able to interpret the results as well.