Résumé : The evolution of the orientation relationship between mechanical twins and the surrounding matrix with the degree of plastic deformation has been characterized at the nanoscale in twinning-induced plasticity steel. The recently developed automated crystal orientation mapping in transmission electron microscopy revealed that the ideal twin relationship is retained up to large levels of strain, while large orientation gradients are built up within the matrix. This particular evolution undoubtedly plays a role in the large work hardening rate of these steels.© 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.