The general ambition of the project is to contribute to a better understanding and prediction of the multilevel interactions between interfaces present in materials and the deformation and failure mechanisms, in order to inspire the development of new materials with extreme structural performances. The field of research sits at the frontier between materials science and solids mechanics. One of the main driving forces for this project is the recognition that the significant progresses recently made in the development of nanostructured materials, especially towards ultra hard systems, must now be integrated into a more hierarchical approach of the material structure involving the control of multiple length scales. This is a necessity for the implementation of a multiproperty vision of materials, considering that several mechanical properties can only be improved if larger length scales are properly architectured. Among the features that can be used to modify and optimize the mechanical properties of materials, interfaces, such as grain, twin and phase boundaries, free or anodized surfaces, play a central role. It is indeed the combined effect of all these interfaces that interact with, trigger, or mitigate the deformation and fracture mechanisms that ultimately control the mechanical response of materials. As a matter of fact, the effect of the presence of interfaces is often larger than the effect of the chemical composition. The focus will be on mechanical properties involving the strength, ductility, fracture, creep, fatigue and wear resistance of metal based systems with a high density of interfaces organized at different scales, possibly involving organic layers. More precisely, the investigations will be performed on 3D bulk materials and on small dimensions systems in the form of thin films or nanowires with a high density of interfaces. Some key scientific issues concern the importance of rate dependent behaviour and back stress originating from the abundance of these interfaces, the stress/strain driven formation and mobility of these interfaces, the interactions with the elementary atomistic deformation mechanisms and with the cracking mechanisms, and the resulting size effects. Meeting this grand ambition will allow unravelling how interfaces can be best organized over a range of length scales in materials with hierarchical structures designed to enforce the adequate mechanisms. The practical outcomes consist in the development of predictive models and of improved processing routes as well as improved characterization methods towards the development of new materials with enhanced performances.