Materials that can withstand extreme conditions such as stresses close to their theoretical strengths, temperatures close to their melting points, highly oxidizing or corrosive environments, and high radiation fluxes are central to improving the performance, safety, and efficiencies of existing energy conversion and utilization systems as well as enabling next-generation ones. When materials are pushed to their limits, microstructural and environmental extremes can have unforeseen consequences, so detailed characterization of defect distributions combined with a deep understanding how they affect materials behavior, is critical to the development of advanced materials of the future. Materials Under Extremes represents the coordination of technical expertise necessary for the development of materials and systems capable of accessing performance regimes beyond their current levels. Our combined fundamental and applied materials science approach draws heavily upon relevant ORNL assets such as first principles modeling and advanced computational materials science resources, cutting-edge analytical equipment in our laboratories, centers and user facilities, and our advanced processing and materials testing infrastructure. As the next-generation technologies envisioned require materials and components to perform under extraordinary conditions, understanding how to engineer materials for superior performance is the focus of our current research.