1. The use of high-temperature metallography and a vacuum stage makes it possible to reveal different mechanisms of austenite grain growth in carbon and alloy structural steels. 2. It was found that austenite grain growth occurs at 850–1250° by three different mechanisms: I) resorption of grains; II) formation of new boundaries and grains; III) boundary migration. The first and second mechanisms are accompanied by decomposition of old boundaries and the third is characterized by movement of the boundaries. 3. Resorption of grains predominates at temperatures 50–100° above Ac3, and the formation of new more even boundaries at temperatures 250–300° above Ac3. Boundary migration occurs at all temperatures but affects grain growth only above 1100°. 4. The resorption of grains evidently depends on rebuilding of dislocation arrays, and the formation of new grain boundaries on rebuilding of dislocation arrays and movement of atoms. The process of boundary migration may be determined by the movement of dislocation groups and collective diffusional movements of atoms.