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The velocities of dislocations have been measured as a function of stress for both undeformed and deformed crystals. The motion is slower at a given stress in a strained crystal, and extra stress is needed to give the same velocity in a strained crystal as in one that is unstrained. It has been found that the extra stress is just equal to the difference in macroscopic flow stresses for the two states of a crystal. Existing theories of strain‐hardening do not correctly predict the behavior of LiF. Barrier‐type theories are ruled out because the distance that a dislocation can move in a strain‐hardened crystal does not seem to be limited. Theories of Taylor hardening or of cutting of a dislocation forest do not give the observed dependence of strain‐hardening on dislocation density. It is observed that the strain‐hardening increment is proportional to the dislocation density (also to the plastic strain), and the proportionality constant is 3–5 dynes/disl. The data are consistent with the idea that the hardening is due to defects left in the wakes of moving dislocations. These defects would interfere with the movement of subsequent dislocations on the same or nearby glide planes.