  We present a variational principle governing the quasistatic evolution of a
linearized elastoplastic material. In case of linear hardening, the novel
characterization allows to recover and partly extend some known results and
proves itself to be especially well-suited for discussing general approximation
and convergence issues. In particular, the variational principle is exploited
in order to prove in a novel setting the convergence of time and space-time
discretizations as well as to provide some possible a posteriori error control.
