The reaction-control-system requirements for the space shuttle include long life, high reliability, reusability, minimal easy system maintenance, and refurbishment in addition to the usual requirements of reasonable system performance, minimum system weight, and flexibility to operate over a wide range of environmental and operational conditions. The use of gaseous-oxygen/hydrogen propellants potentially provides this capability. Identifiable space-shuttle reaction-control-system requirements are presented, and the two most promising system concepts described and compared. These concepts are the low-chamber-pressure concept, in which the main injection tanks are used as accumulators and heat exchangers to provide the reaction-control engines with the gaseous propellants; and the high-chamber-pressure concept, in which a gas generator/turbopump/heat exchanger is used to provide the propellants to the engines. The overall performances of the two systems are compared and results of sensitivity studies presented. Results of the Manned Spacecraft Center in-house oxygen/hydrogen reaction-control-system-engine technology program are examined, including performance evaluation (combustion efficiency, heating rates, etc.) of three basic injector concepts. Significant factors that influence the engine design relative to application to the space shuttle are discussed.