Only recently has increasing use been made of computer simulation in the field of extruder engineering. This simulation is based on physico-mathematical process models. In order to achieve a closed, process engineering-based extruder layout and operating behavior simulation it is necessary to have models describing the throughput behavior, the melting process, the temperature development. the pressure profile over the screw length, the residence time distribution, the mixing and dispersion processes, and also the torque and power requirements. In the Plastics Technology Section of the University of Paderborn (KTP) a program for the computer-aided design of extruders (R(E)X), financed by 17 companies, was developed which will permit rapid application of the models developed there and will fulfill the requirements set out above. Since no computation-intensive numerical methods like Finite Element or Finite Difference Methods (FEM, FDM) are required, the algorithms are very rapid, which means that short computing times are achieved (just a few seconds on an IBM AT).