ABSTRACTA review is presented of the work on the influence of impurities on the dynamic behavior of dislocations in silicon crystals and also on the resulting effects in the mechanical strength performed by the author's group. Special emphasis is laid on the effects of light element impurities such as oxygen, nitrogen and carbon. Although all of these impurities do not affect the velocities of dislocations moving under relatively high stresses, oxygen and nitrogen atoms are found to lock slowly moving dislocations and dislocations at rest very effectively. Such locking of dislocations results in the decreases in the activities of generation and multiplication centers for dislocations, leading to the strengthening of the crystals. The high strength of silicon crystals brought about by the impurities is lost when the crystals are subjected to the heat treatments which allow the precipitation of the impurities on a macroscopic scale. This softening is shown to be caused by dislocations punched out from precipitates and by the exhaustion of the impurities dissolved in the matrix crystal.