Based on the experimental results obtained mainly by the authors, the effect of decreasing the temperature to 77 K using various methods of cryogenic severe plastic deformation (cryo-SPD), such as equal-channel angular pressing, high-pressure torsion, as well as large deformations in the form of cryorolling, on the mechanical properties and mechanisms of plastic deformation is investigated. The results are presented for deformation of a number of metals (titanium, zirconium, cobalt), Ni–18.75%Fe alloy and high-entropy alloys (HEA) Al_0.5CoCrCuFeNi, CrMnFeCoNi₂Cu, CoCrFeNiMn over a wide range of low temperatures. The efficiency of using cryo-SPD methods to increase their strength is demonstrated. The reasons for the paradoxical effect of cryo-SPD on the mechanical characteristics of some HEAs, which contradict the thermally-activated nature of plastic deformation, are analyzed. It is shown that the use of the thermal activation analysis method makes it possible to establish the most probable physical mechanisms that determine the plasticity of a wide class of materials in the coarse-grained and nanostructured states. The features of plastic deformation of various metals and alloys in the coarse-grained and nanostructured states in the ultralow temperature range (4.2–0.5 K) are described.