Post-extrusion swelling and extrudate distortion phenomena reveal the viscoelastic nature of elastomeric materials and are particularly critical in rubber processing. With the short dies used in the rubber industry, it is clear that extrudate swell depends not only on the rheological properties of the compounds, but also on the particular geometry of the extrusion system. Consequently, it is very difficult to compare laboratory results with the experience of industrial practice, because of the number and variety of extrusion processes. However, it is possible to draw some generally valid conclusions from the data currently available. The main factors affecting post-extrusion swelling and the critical conditions for the onset of extrudate distortion are: (1) the elastomer, its nature and molecular characteristics; (2) the filler, its grade and level; (3) the (cumulative) energy introduced into the rubber during the mixing operations; (4) the mixing procedure (particularly in blend preparation); (5) the temperature and shear conditions of the extrusion process. The use of sophisticated test instruments, such as the MPT enables the processability parameters of a compound to be rapidly obtained, which can often be used as a key to solve practical plant problems.