Temporal Order in Dirty Driven Periodic Media

We consider the non--equilibrium steady states of a driven charge density wave in the presence of impurities and noise. In three dimensions at strong drive, a true dynamical phase transition into a temporally periodic state with quasi--long--range translational order is predicted. In two dimensions, impurity induced phase slips are argued to destroy the periodic ``moving solid'' phase. Implications for narrow band noise measurements and relevance to other driven periodic media, e.g. vortex lattices, are discussed.