Reaction injection molding (RIM) is the high-speed production of polymer parts by quick injection of low viscosity reactive monomers or prepolymers into complex shaped cavities. The trial and error approach reflects the state of the art in this relatively new process. Obviously, material and process parameters determine the 'moldability' of a specific system in a particular application. The concept of 'molding areas' on the critical parameters plane can be extended from thermoplastic injection molding to RIM. These moldability diagrams are intended to give first guidelines for process optimization as well as directions for starting new materials and new mold cavities.