Abstract : Derating can be defined as the practice of limiting electrical, thermal and mechanical stresses on devices to levels below their specified or proven capabilities in order to enhance reliability. If a system is expected to be reliable, one of the major contributing factors must be a conservative design approach incorporating part derating. Realizing a need for derating of electronic and electromechanical parts, many manufacturers have established internal guidelines for derating practices. The Air Force, on the other hand, has no established guide or base line for evaluating the validity of the numerous deratings proposed by industry. Therefore, the objective of this effort was to develop and publish guidelines for part derating to be used as standards for evaluating contractor's design and to establish values to be implemented in system and equipment specifications. This document has established part derating levels based on mission critically for the majority of devices included in MIL- HDBK-217. Part design application guidelines were also developed. This study indicated that some advanced technology devices (VLSI, bubble memory, microwave semiconductors, etc.) has little or no available derating data and will require a more in-depth follow on report. (Author)