Stress relaxation processes are considered in spring alloys hardened by thermomechanical treatment. Structural transformations of these alloys are tabulated according to causes of their initiation and to their responsibility for stress relaxation development. The analysis of stress relaxation mechanism allows to formulate a principle of designing spring alloys with high strength and resistance to stress relaxation. This resulted in development of austenitic strain-aged steels. For high temperature applications it is proposed to use springs of single crystals.