One of the most critical elements in the transition of a structural material from the development stage to an aerospace application is the existence of reliable design allowables. The Department of Defense (DOD) and the Federal Aviation Administration (FAA) require the use of MIL-HDBK-5 (for metals) and MIL-HDBK-17 (for composites) allowables unless alternate values are approved by these organizations. The major benefit of handbook allowables is not, however, the automatic approval of a material for use. It is the reliability and confidence that comes to the designer in being able to use a material with predictable property values. The end result is a design that is safe yet not overly burdened with excess structure. This article reviews the requirements for the development of allowables and some of the potential problems which may be encountered in developing allowables for metal matrix composites.