Heterogeneous materials are a new class of materials with unprecedented mechanical properties, combining high strength and ductility that are not achievable in homogeneous materials. These materials consist of domains with significant differences in strength, ranging from micrometers to millimeters in size. During deformation, large strain gradients occur near domain interfaces, generating significant back-stress that strengthens the material and enhances ductility through work hardening. High interface density is essential for maximizing back-stress, which is a new strengthening mechanism. Heterogeneous materials can be defined by their dramatic strength heterogeneity, caused by microstructural, crystal, or compositional differences. The deformation behavior of these materials is divided into three stages: in stage I, both soft and hard domains deform elastically; in stage II, soft domains start to deform plastically while hard domains remain elastic, creating mechanical incompatibility and strain gradients; in stage III, both domains deform plastically, with soft domains sustaining higher strain, leading to strain partitioning and enhanced work hardening. Back-stress, generated by geometrically necessary dislocations, plays a crucial role in the mechanical behavior of heterogeneous materials. It can be measured experimentally and is linked to plastic strain gradients. The physical origin of back-stress is the piling-up of dislocations at domain boundaries, which creates a long-range stress that counterbalances applied stress. To achieve optimal mechanical properties, heterogeneous structures should have high domain interface density and large spacing to allow effective dislocation pile-up. Strain partitioning among domains increases strain gradients and back-stress work hardening, enhancing ductility. The heterogeneous lamella structure, with soft domains embedded in a hard matrix, is considered near-ideal due to its high strength and ductility. Heterogeneous materials are a fast-emerging field with potential for significant advancements in mechanical properties. They offer a new approach to achieving strength-ductility synergy, surpassing traditional methods. The field is growing rapidly, with increasing research and international conferences, making it a promising area for future development.Heterogeneous materials are a new class of materials with unprecedented mechanical properties, combining high strength and ductility that are not achievable in homogeneous materials. These materials consist of domains with significant differences in strength, ranging from micrometers to millimeters in size. During deformation, large strain gradients occur near domain interfaces, generating significant back-stress that strengthens the material and enhances ductility through work hardening. High interface density is essential for maximizing back-stress, which is a new strengthening mechanism. Heterogeneous materials can be defined by their dramatic strength heterogeneity, caused by microstructural, crystal, or compositional differences. The deformation behavior of these materials is divided into three stages: in stage I, both soft and hard domains deform elastically; in stage II, soft domains start to deform plastically while hard domains remain elastic, creating mechanical incompatibility and strain gradients; in stage III, both domains deform plastically, with soft domains sustaining higher strain, leading to strain partitioning and enhanced work hardening. Back-stress, generated by geometrically necessary dislocations, plays a crucial role in the mechanical behavior of heterogeneous materials. It can be measured experimentally and is linked to plastic strain gradients. The physical origin of back-stress is the piling-up of dislocations at domain boundaries, which creates a long-range stress that counterbalances applied stress. To achieve optimal mechanical properties, heterogeneous structures should have high domain interface density and large spacing to allow effective dislocation pile-up. Strain partitioning among domains increases strain gradients and back-stress work hardening, enhancing ductility. The heterogeneous lamella structure, with soft domains embedded in a hard matrix, is considered near-ideal due to its high strength and ductility. Heterogeneous materials are a fast-emerging field with potential for significant advancements in mechanical properties. They offer a new approach to achieving strength-ductility synergy, surpassing traditional methods. The field is growing rapidly, with increasing research and international conferences, making it a promising area for future development.