This paper is written to honor Dr. George R. Irwin and reviews several key developments in fracture mechanics based on his “stress-intensity factor” concept. The early development of two fundamental crack solutions, (1) an edge crack in a semi-infinite body and (2) the surface crack, are highlighted. Applications of Irwin's early concepts by other researchers to characterize fatigue-crack growth and brittle fracture of metallic materials are presented. The stress-intensity factor is the cornerstone of the damage-tolerance and durability design concepts used by the aerospace community around the world. The stress-intensity factor concept, crack-closure mechanics, and the observation that “fatigue is crack propagation” in many engineering materials has led to a merger of fatigue and fracture mechanics analysis methodologies. Irwin's recognition of the importance of the normal stress parallel to the crack (now referred to as the T-stress) in fracture led many to propose a two-parameter characterization for fracture. The importance of constraint on crack-tip yielding has been further advanced by the use of high-powered computers to calculate a normal-stress constraint parameter following his ideas. The father of fracture mechanics has left a legacy that will endure and provide safer and more reliable structures in the future.