Gradient models have been intensively discussed in the literature for the study of time- dependent or time-independent processes such as visco-plasticity, plasticity and damage. This paper is devoted to the theory of Gradient Plasticity. A general and consistent math- ematical description available for common time-independent behaviours is presented. Our attention is focussed on the derivation of general results such as the description of the gov- erning equations for the global response, for the rate response, the expression of the associ- ated variational principles and the question of uniquenness in terms of the energy potential ́ and the dissipation potential.