Custom 465 Stainless alloy is a recently introduced PH stainless steel designed to provide the highest combination of strength and toughness commercially available. It was designed for use in the aerospace industry, as a higher strength replacement for other PH stainless grades such as 13-8 and 15-5, or as a corrosion resistant replacement for alloy steels such as 4340. However, in order for a new alloy to find acceptance in this industry, it must undergo sufficient testing to provide potential users with a comfort level as to its capabilities. Due to the huge potential for liability if an inadequate material is put into service, the acceptance of a new alloy for aerospace applications is often a long and drawn-out process. This industry requires an alloy manufacturer to conduct testing far beyond what is required for other industries. The work and expense that had to be undertaken by the alloy manufacturer to have the alloy considered for use in the aerospace industry, as well as additional testing that is normally conducted by the aerospace manufacturer, will be discussed. Carpenter Custom 465 alloy has undergone the testing required to provide AMS and ASTM specifications, as well as MIL-HDBK-5J design allowables. MIL-HDBK-5 testing was performed on 10 heats, in sizes ranging from 3” square to 12” round. Testing included tensile, compression, shear, and bearing properties for both the H950 and H1000 conditions. Overall, in excess of 1000 individual samples were tested as part of this program. In addition, more than 150 individual CVN and fracture toughness samples were tested. Test results and MIL-HDBK-5 design allowables will be presented. Because of this extensive testing, data are now available to provide aerospace manufacturers and designers with the property minimums required to consider the use of this new alloy.