This special issue of Materials Performance and Characterization features seven papers from the 23rd International ASTM/ESIS Symposium on Fatigue and Fracture Mechanics, held November 12–13, 2025, in Atlanta, Georgia. Sponsored by ASTM International Committee E08 on Fatigue and Fracture and co-sponsored by the European Structural Integrity Society, the symposium focused on environmental effects on fatigue and fracture of structural materials, providing a forum for the exchange of ideas, test methods, data, and analysis approaches related to fatigue and fracture in aggressive environments. Topics included environmentally assisted cracking, corrosion fatigue, hydrogen embrittlement, cryogenic and elevated-temperature behavior, pipeline materials, and industrial applications of fatigue and fracture assessment. The papers in this issue reflect the breadth of the symposium, beginning with the Swedlow Award Lecture on elevated-temperature fatigue crack growth models for nickel-base superalloys. Additional contributions address fracture toughness assessment of ferritic ductile cast iron, near-threshold fatigue crack growth at elevated temperature, environment-induced cracking in high-strength aluminum alloys, fatigue life and texture effects in pipeline steels, and corrosion fatigue crack propagation modeling for industrial pump components. Together, these papers highlight current advances in understanding, testing, and modeling environmental durability in materials and components critical to energy, transportation, infrastructure, and other demanding engineering applications.