“Spinning Mass” Inertial Measuring Units (SMIMU’s) such as gyroscopes and accelerometers comprise a proven technology that completes their tasks with remarkable levels of precision. This stringent precision requirement has necessitated the use of a number of critical materials, most notably beryllium, which have existed almost unchanged since the late 1950’s. A brief overview of the operating principles of SMIMU’s is given. System-critical materials such as beryllium, cemented carbides, copper alloys, Inconels, and age-hardened stainless steels are reviewed, as are the properties which necessitate their use in SMIMU’s. Examples are given of the procedures employed in order to keep stockpiles at acceptable levels.