This perspective discusses the concept of back stress in heterostructured (HS) materials and proposes a new term, hetero-deformation induced (HDI) hardening, to describe the strengthening mechanism. HS materials, which consist of both soft and hard domains with different flow stresses, exhibit enhanced mechanical properties due to interactions between back stress and forward stress. Back stress, which is generated in soft domains to offset applied stress, makes them appear stronger, while forward stress, generated in hard domains, makes them appear weaker. The extra hardening in HS materials is attributed to the interaction between these two stresses, and should be described as HDI hardening. The measured 'back stress' in the literature is redefined as HDI stress. Back stress is often associated with the Bauschinger effect and kinematic hardening, but its physical origin is not well understood. Dislocation models show that back stress is produced by geometrically necessary dislocations (GNDs), which create lattice bending or misorientation. There are two basic types of GND arrangements: Type I, which produces back stress, and Type II, which does not. Mixed arrangements may exist, but are less effective in producing back stress. Back stress plays a critical role in the strengthening of HS materials, but its measurement from mechanical testing is challenging. The measured 'back stress' is not the true back stress, as it is influenced by forward stress. The unique mechanical behavior of HS materials is due to the interaction between back stress and forward stress, and cannot be attributed to back stress alone. The new term HDI hardening is proposed to describe the strengthening mechanism, with HDI stress being the measured 'back stress'. Future research should focus on understanding the relationship between back stress, forward stress, strain gradient, and HDI stress, as well as the fundamental physics of HS materials.This perspective discusses the concept of back stress in heterostructured (HS) materials and proposes a new term, hetero-deformation induced (HDI) hardening, to describe the strengthening mechanism. HS materials, which consist of both soft and hard domains with different flow stresses, exhibit enhanced mechanical properties due to interactions between back stress and forward stress. Back stress, which is generated in soft domains to offset applied stress, makes them appear stronger, while forward stress, generated in hard domains, makes them appear weaker. The extra hardening in HS materials is attributed to the interaction between these two stresses, and should be described as HDI hardening. The measured 'back stress' in the literature is redefined as HDI stress. Back stress is often associated with the Bauschinger effect and kinematic hardening, but its physical origin is not well understood. Dislocation models show that back stress is produced by geometrically necessary dislocations (GNDs), which create lattice bending or misorientation. There are two basic types of GND arrangements: Type I, which produces back stress, and Type II, which does not. Mixed arrangements may exist, but are less effective in producing back stress. Back stress plays a critical role in the strengthening of HS materials, but its measurement from mechanical testing is challenging. The measured 'back stress' is not the true back stress, as it is influenced by forward stress. The unique mechanical behavior of HS materials is due to the interaction between back stress and forward stress, and cannot be attributed to back stress alone. The new term HDI hardening is proposed to describe the strengthening mechanism, with HDI stress being the measured 'back stress'. Future research should focus on understanding the relationship between back stress, forward stress, strain gradient, and HDI stress, as well as the fundamental physics of HS materials.