Severe plastic deformation of metals leads to changes in microstructure, which in turn are responsible for unusual bulk properties. There are two (interrelated) processes leading to change of microstructure: 1) the production of a very high number of dislocations, leading to a high dislocation density. 2) the "fragmentation" of the original crystal grains into much smaller structural elements leading to what is sometimes called "nanocrystal line" material. Under continued strain neither of these two processes will continue indefinitely. Instead, the microstructure will reach a steady state. Under certain conditions its development may even be reserved. This indicates the operation of restoration mechanisms such as dynamic recovery and recrystallisation. The actual state of art will be discussed on the basis of current experiments.