A lecture given by Francisca Garcia Caballero, at the Adventures in the Physical Metallurgy of Steels (APMS) conference held in Cambridge University. The atomic mechanism of the bainite transformation is discussed in the context of the highest resolution analytical experiments conceivable. After decades of debate on the mechanism for the formation of bainite, it is accepted that bainite grows via a displacive mechanism i.e., as plate-shaped transformation products exhibiting an invariant plane strain surface relief effect. But there is still much discussion on the diffusion or diffusionless nature of bainite. Elements of the theory are now routinely being used in the design of innovative steels and in the interpretation of a variety of experimental data. However, current experimental and theoretical understanding is limiting technological progress. The purpose of this atom probe tomography study was to track the atom distributions during the bainite reaction in steels with different carbon and silicon contents transformed over a wide range of temperatures (200-525 centigrade) to elucidate the role of reaction rate and diffusion in the formation of bainite with and without cementite precipitation. The results are providing new experimental evidence on subjects critically relevant to the understanding of the atomic mechanisms controlling bainitic ferrite formation, such as the incomplete transformation phenomenon, the carbon supersaturation of ferrite, the plastic accommodation of the surrounding austenite and cluster and carbides formation.