Generality and distinctions in the mechanism of superplastic (SP) deformation between metals and ceramic materials are analyzed. It is established, that SP deformation depends on properties of grain boundaries and is determined by their structure and long-range area in a polycrystal. Under optimal SP deformation conditions there operates a specific mechanism of deformation, namely, cooperative grain boundary sliding (CGBS). The possible mechanisms of formation of CGBS bands are considered. Difference in SP behavior of metallic and ceramic materials is connected with occurrence of liquid and amorphous phases at grain boundaries during deformation.