The paper includes a brief description of a typical modern extrusion line used for the manufacture of cable components, together with the reasons why instrumentation is necessary. Foremost among these is the maintenance of product quality, which is discussed in terms of (a) meeting precise dimensional and electrical requirements, (b) the need for indicating to the operator any trends away from desired values, and (c) the saving of materials. A distinction is made between the processes of core insulation and cable sheathing and the respective instrument accuracy and stability limits required.In addition, the arrangements of a number of systems which are commonly employed to monitor the various parameters are described and their advantages and disadvantages discussed. The reasons for the choice of the types adopted in particular applications are also given.Also considered are the use of instruments described as elements of automatic control systems for extrusion lines and the difficulties encountered.