Service failures have occurred in a number of aircraft parts made of quenched and tempered steel heat treated to ultimate tensile strengths of 260,000 to 280,000 psi. Some of these failures have been attributed to “delayed cracking” as a result of hydrogen embrittlement or to stress-corrosion. Because of the serious nature of the failures and because the mechanism of the fracture initiation is not well understood, unusually complete laboratory investigations have been conducted. Three of these investigations are reviewed to illustrate the methods used in studying failures in aircraft parts. The results of the laboratory studies indicate that unusual care is necessary in the processing and fabrication of ultra-high-strength steel and in the design and maintenance of the structures in which it is used.