This paper emphasizes the requirements peculiar to spacecraft fluid mechanical systems. Although the design requirements for the fluid power systems used in aircraft, tactical missiles, and launch vehicles have been established successfully, criteria for the design of gaseous and liquid systems in spacecraft demand additional considerations. Elements of fluid and mechanical systems in manned and unmanned spacecraft are discussed, and additional visibility is provided for those design requirements that differ from earth-oriented vehicles. Of particular interest are the newest spacecraft under design-the space shuttle (a recoverable booster and orbiter) and Viking. The material suggests new design criteria with special considerations for fluid systems and other operating mechanisms exposed to a space environment. It is anticipated that such topics as environment, materials, lubrication, zero gravity and inflight maintenance, as related to the design of fluid and mechanical systems, will generate new interest among members of the SAE-A6.