New experimental data related to the grain size and the Bauschinger effects were obtained from a high-Mn austenitic twinning-induced plasticity steel. As the data showed a strong contribution of kinematic hardening to the mechanical behaviour, a new physical-based model describing the isotropic and kinematic hardening was presented and validated in relation to the grain size and the twin spacing during straining.Effect of Grain and Twin Boundaries on the Hardening Mechanisms of Twinning-Induced Plasticity Steels. O.Bouaziz, S.Allain, C.Scott: Scripta Materialia, 2008, 58[6], 484-7