During rolling contact fatigue of ball bearings structural changes may occur below the raceway, in the region of maximum shear stress. A great number of 6309 type bearings were tested at two stress levels for varying numbers of revolutions, and the resulting structural changes in the inner rings were studied by optical and electron microscopy. The present observations have been compared to previously reported observations, which are often described in unsystematic terminologies and sometimes apparently contradictory. A unified terminology is worked out, on the basis of which, the structural changes are described. The following features occur, in chronological order. i) A ferritic phase, containing an inhomogeneously distributed excess carbon content, corresponding to that of the parent martensite. A mixture of this phase with residual martensite constitutes the well known dark etching region. ii) Disc-shaped regions of ferrite, thermodynamically stable, about 0.1 μm thick, inclined by about 30 deg to the raceway, and sandwiched between carbide rich discs. The latter are constituted by very small carbide particles, and are not necessarily compact. iii) A second set of larger disc-shaped regions about 10 μm thick, of plastically deformed ferrite in thermodynamic equilibrium, forming an angle of about 80 deg to the raceway. Transformation mechanisms are proposed. Particular attention is paid to short range carbon diffusion, induced by the cyclic stresses.