Evidence is presented in this paper to show that crystals present a pla8tic 'resistarwe to dislocation motion. This has been demonstrated by direct observations of dislocations in LiF crystals by Johnston and Gilman, and in Fe-Si crystals by Stein and Low. The plastic resistance is almost independent of dislocation velocity except at very high velocities. It is, however, strongly influenced by temperature, impurities, and radiation damage. In LiF crystals, the plastic resistance is much higher for glide on {100} planes than on {nO} planes.