Metal working processes encompass a wide range of strain, strain rates and temperatures. Strains range from less than 0.01 (for example in skin-pass rolling of interstitial free steel) up to around 1 (cold rolling of strip, extrusion, etc.). Typical strain rates and temperatures are given in Table 11.1 (which is partly an extract from Frost and Ashby (1982). For plastic forming processes the most important characteristics of the material are: the ability to distribute strains; the deformation resistance; and the resulting properties of the formed part. The ability to distribute strains is mainly governed by the work hardening and strain-rate sensitivity. It is also affected by the strain path. The purpose of this paper is to outline the, in our view, most essential material properties for metal working processes and the microstructural reasons for them. We recognise that the presence and development of crystallographic texture is quite an important part but our purpose is not to give an extensive description of this, only to point out some consequences. For further reading we recommend a book by Kocks, Tomé and Wenk (1998). Another aspect that is only briefly covered is the influence of stress states and changes in strain paths during processing or between consecutive process steps.