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The object of this paper is to try to analyse, and also to some extent to justify, the temperature control requirements of the screw extrusion process. This paper and the following paper on ‘Design Strategies for the Control of Temperature’ will, we hope, be of interest both to the section of the Plastics Industry that uses extruders, and also to that part of the Instrumentation Industry which is concerned with the control of temperature. We believe that, in sectors of both industries, there are still many misconceptions as to the nature of the extrusion process and about the equipment used. To start with, we do not believe that the Plastics Industry knows the standard terms used by the Instrumentation Industry to describe the different types of instrument they employ. We hope Table I will help to clarify this situation. We also believe that the Instrumentation Industry has a limited understanding of the extrusion process, and so before describing what we believe to be the requirements for temperature control in extrusion, we will survey briefly the materials, products, properties, tolerances and also the equipment associated with this process.