Rheology is a powerful tool that can be used to relate polyolefin rheological behavior to molecular structure and, furthermore, to “fingerprint” polyolefin behavior during processing. This presentation will demonstrate the power of rheology in polyolefin characterization. In particular, extensional flow measurements generate highly directional flow fields that are very sensitive to the molecular structure of a polymeric system. In one case, extensional flow and peel results in the melt state provide insight into the origins and mechanisms of flow instabilities such as melt fracture.