Extrusion is one of the most important operations in the polymer-processing industry. In order to improve their productivity, polymer processors must rely on computer-aided design, engineering, and manufacturing (CAD/CAE/CAM). The performance of an extruder is governed by its capacity to convey, melt, and pump the polymeric material. In the past, a great deal of effort, theoretical and experimental, has been put into the analysis of plasticating single-screw extruders. Despite these developments, the perfect model that could help the practicing polymer engineer to design extruder screws does not exist. A comprehensive model has been developed for steady-state computer simulation of a single-screw plasticating extruder (barrier and conventional screws) in order to predict the performance of the extruder, the optimum screw design, and the operating conditions. The model takes into account the three zones of the extruder: the solids-conveying section, the plasticating section, and the metering section.