This paper describes propulsion systems under consideration for orbital maneuvering stages (Space Tug or Orbit-to-Orbit Shuttle). It describes the Space Tug/Orbit-to-Orbit Shuttle (OOS) configurations, and the missions to be performed by the Space Tug/OOS. The propulsion system requirements for the Space Tug/OOS main engine and the attitude control systems are described. The effect of the mission and operational requirements on the main engine system characteristics (for example, start, stop, throttling, reuse, life, etc.) are described. A listing of the propulsion system candidates for this application is provided. Briefly described are existing systems such as Delta, Agena, Centaur, and Transtage. New engine systems (the Staged Combustion engine and the Aerospike engine) for this application are described in greater detail. This description is followed by a definition of the technology status for both the Staged Combustion engine and Aerospike engine, which are the leading candidates for this application.