The advancement of structural materials with different engineering applications is dependent onunderstanding their mechanics and mechanisms of deformation and fracture. The performance of materials under extreme environmental conditions is the limiting factor in many engineering systems: from the jet engine, to nuclear power plants, to something as mundane as a bridge or building. In this study, three structural materials are examined from the perspective of understanding how their unique microstructures lend them their ability to mechanically withstand the extreme environments they are designed for. First, Deformed and Partitioned (D&P) steel is discussed. D&P steel possesses good mechanical properties: a yield strength around 2GPa and a fracture toughness as high as 100 MPa√m. The way D&P steel is processed produces a highly tailored martensite/austenite duplex microstructure. This microstructure allows for both transformation induced plasticity (TRIP) and delamination toughening to forego the strength-toughness trade-off, without the addition of expensive alloying elements. Next, Tristructural isotropic (TRISO) nuclear fuel particles tested with the ALS tomography beamline are described. Due to the complex layered microstructures, in situ tomography is required to examine the internal features and failure mechanisms of the particles while being deformed at room temperature and 1000 ℃. This technique is used to examine the change in failure loads and fracture mechanisms due to the presence or absence of a SiC layer within the particles. For the TRISO particles, the results show that the SiC layer is responsible for a decrease in strength at higher temperatures due to the relaxation and redistribution of residual stresses. Finally, the mechanical properties, microstructural characterization, and failure mechanisms of body-centered cubic refractory high entropy superalloys (Ti20Zr20Nb25Ta25Al10) are provided for two differing heat treatments. The first heat treatment has a microstructure with a brittle matrix and a ductile precipitate, whereas the second is inverted, having a ductile matrix and brittle precipitate. These two heat treatments were then examined in compression, tension, and fracture toughness at room and elevated temperatures. These materials have high yield strengths and ductility in compression, yet they are brittle in tension and have low fracture toughness values at all temperatures. Both heat treatments were brittle in tension and failed intergranularly because a ductile phase with a smaller fraction of the secondary strengthening precipitate phase formed at the grain boundaries which weakened the material.