The phenomenon of softening of crystalline materials by alloying, which is promising for controlling mechanical properties of materials, is due to the occurrence of an additional channel of facilitated formation of dislocation kinks at impurity centers. This effect is related to the kink mechanism of dislocation motion. Hence, a range of materials that are capable of softening can be singled out: metals with a bcc structure, semiconductors, ceramic materials, intermetallic compounds, etc. A unified basis for description of softening regularities is given by the phenomenological theory. This theory predicts many properties that are general for all materials under consideration: the range of strength characteristics of impurities capable of softening crystals, limiting possibilities of softening at the optimal choice of the components, etc. The theory, supplemented with the knowledge of some material constants (determined from microscopic calculations or, even better, from experiment), makes it possible to calculate the temperature and concentration dependences of the yield stress and other parameters measured in mechanical tests.