This review encompasses fundamental principles and rheological equations of state, polymer melt rheology (shear and extensional flow, viscoelasticity, die swell and melt fracture) and rheological measurement techniques. It describes the main plastics processing techniques, and explains the influence of polymer melt rheology upon their operation. 48 figures and more than 80 equations enhance the review, which is also supported by an extensive, indexed bibliography, containing over 400 references and abstracts selected from the Polymer Library.