The flow of material during injection upsetting was studied to identify areas of the process which would require either, detailed definition or the use of simplifying assumptions, to enable the development of a complete mathematical model. Equipment for injecting El CM aluminium on to an upsetting platen and into a die-cavity, and a plane-strain equipment for injecting slabs of plasticene, were used to examine the characteristics of sliding in the injection chamber, of flow at the exit and of flow into the die-cavity. Considerations which are critical to the modelling of injection upsetting are the friction losses within the injection chamber, the presence of regimes of flow behaviour which are bounded by specific values of flange thickness and the identification of the extent of the transitional flow prior to the establishment of a steady-state distribution of shear surfaces. The accommodation of the strain-dependent behaviour of materials requires a fundamental description of strain within the upset flange.