The article discusses the concept of hetero-deformation induced (HDI) hardening and the redefinition of "back stress" in the context of heterostructured (HS) materials. HS materials, which exhibit superior mechanical properties due to their heterogeneous microstructures, have been the subject of extensive research. The authors argue that the traditional concept of "back stress," which is often used to explain the strengthening and extra strain hardening in HS materials, is inadequate. They propose that this "back stress" should be renamed HDI stress to accurately reflect the interaction between back stresses and forward stresses developed in soft and hard domains, respectively. The article reviews the history of the back stress concept, its theoretical underpinnings, and its measurement methods. It highlights that back stress is typically small in homogeneous materials but becomes significant in HS materials due to the dramatic variation in flow stresses across different domains. The authors explain how geometrically necessary dislocations (GNDs) pile up against domain boundaries, producing back stress in soft domains and forward stress in hard domains. This interaction leads to extra strain hardening and enhanced strength. The authors also address the limitations of the back stress concept, noting that it does not fully explain the unique mechanical behavior of HS materials. They propose that the interaction between back stress and forward stress, rather than their sum, should be considered. The article concludes by defining HDI hardening and HDI stress, and outlines several outstanding issues that need further investigation, including the interaction between back and forward stresses, the behavior of HDI stress during deformation, and the role of GNDs in strain gradient accommodation.The article discusses the concept of hetero-deformation induced (HDI) hardening and the redefinition of "back stress" in the context of heterostructured (HS) materials. HS materials, which exhibit superior mechanical properties due to their heterogeneous microstructures, have been the subject of extensive research. The authors argue that the traditional concept of "back stress," which is often used to explain the strengthening and extra strain hardening in HS materials, is inadequate. They propose that this "back stress" should be renamed HDI stress to accurately reflect the interaction between back stresses and forward stresses developed in soft and hard domains, respectively. The article reviews the history of the back stress concept, its theoretical underpinnings, and its measurement methods. It highlights that back stress is typically small in homogeneous materials but becomes significant in HS materials due to the dramatic variation in flow stresses across different domains. The authors explain how geometrically necessary dislocations (GNDs) pile up against domain boundaries, producing back stress in soft domains and forward stress in hard domains. This interaction leads to extra strain hardening and enhanced strength. The authors also address the limitations of the back stress concept, noting that it does not fully explain the unique mechanical behavior of HS materials. They propose that the interaction between back stress and forward stress, rather than their sum, should be considered. The article concludes by defining HDI hardening and HDI stress, and outlines several outstanding issues that need further investigation, including the interaction between back and forward stresses, the behavior of HDI stress during deformation, and the role of GNDs in strain gradient accommodation.