Driving force calculations for Fe-C, Fe-X-C, and Cu-Zn alloys show that the formation of bainite by a shear mechanism is thermodynamically impossible. There exist superledges on the broad faces of bainite in steels, revealing that the thickening of bainite probably proceeds by a ledge mechanism. In some Fe-Ni-C alloys and commercial steels, no simple relationship was found between the strength of austenite and theBs temperature; however, there is a linear relationship betweenBs and the diffusion coefficient of carbon and iron in austenite, as well as between the incubation period and the function containing DFeγ. The bainite reaction seems to be controlled by diffusional processes. In a low-carbon Ni-Cr steel, the morphology of the bainite/matrix interface boundary is different from that of the martensite, and the habit plane of the bainite (1 7 11)α deviates 13.3 deg from that of the martensite (1 1 0)α, indicating that the mechanism of the bainite reaction is not necessarily analogous to that of the martensitic transformation. At temperatures nearMs, as the driving force will be large enough, the growth of bainite by shear may be able to occur, and evidence is given by the morphology of bainite showing shear characteristic. X-ray diffraction study in Ag-Cd and internal friction measurements in Ag-Cd and Cu-Zn-Al alloys, 18CrNiWA steel, and its decarburized specimen reveal that the nucleation process occurs within the incubation period of bainite formation.