IRG 1 will develop the fundamental science needed to understand, describe, and predict interfacial phenomena in metals and ceramics with multiple principal elements. These so-called complex concentrated materials have been reported to have outstanding properties such as high strength, tailored band gaps, extremely large dielectric constants, and substantially reduced thermal conductivity, making them the next paradigm shift in structural and functional materials. This IRG’s interdisciplinary team will be the first to develop the core principles of microstructural engineering for complex concentrated materials, including fundamental investigations of atomic-level structure and chemistry, interfacial thermodynamics, kinetics, and mechanical and functional properties. This foundational knowledge will then be used to design and synthesize materials with planned microstructures and properties. The team provides complementary expertise in materials theory, computational materials science, processing science, advanced characterization, property measurement, and artificial intelligence to enable a complete study of interfacial behavior in complex concentrated materials. This IRG is expected to transform complex concentrated materials from laboratory curiosities into materials that alter our global economy in a variety of essential industries.