Prognostics and health management (PHM) can provide remarkable insight for maintenance management of large systems, but must be implemented with a healthy respect for the end user and a practical view of the hardware and software capabilities. The F-35 Joint Strike Fighter (JSF) program is implementing a comprehensive PHM system to maximize the supportability of the air system. The prognostic algorithms must have a defined minimum capability to aid implementation and verification. However, the complexity of the air system precludes creating exact requirements for remaining useful life and confidence. Instead, "usefulness criteria" are created to link the user need with the minimum capability of the algorithm. The usefulness criteria are a list of goals related to aircraft supportability which can be used to define the minimum acceptable time to maintenance indication for the prognostic algorithm. The goals in the usefulness criteria were applied to each prognostic algorithm in the F-35 PHM system. When assigned, these usefulness criteria provide a means to measure the improved performance of the aircraft and fleet maintenance as well as prioritize the implementation of the prognostic algorithms. The development and implementation of the algorithms in relation to these usefulness criteria are still in process, but it is expected that most algorithms exceeds the criteria. Those which do not meet the criteria are re-evaluated with a trade study to determine if further efforts in hardware and software development are warranted. This process of usefulness criteria development and application can be rigorously applied to the development of any PHM system. This paper covers the development of the usefulness criteria for the F-35 program and the implementation results to date.