Rheological testing of thermoplastic materials can be performed on both solid samples and on melts. This is important because the choice of material form and physical state is more than a matter of convenience: Product performance issues usually are related to solid samples properties; process ability issues can be correlated with polymer melt properties. In either case, since rheology is an indirect and non-unique measurement of structure, the application of rheology to solving processing and product performance problems often reduces to comparative analysis of good and poor performing samples.