Orbital Technologies Corporation (ORBITEC) and the University of Alabama at Huntsville (UAH) propose to develop the Advanced Microwave Electrothermal Thruster (AMET), a high-efficiency thruster which will use water propellant to enable various Lunar and Mars missions. The proposed AMET will incorporate a number of innovations to dramatically improve upon existing designs, including the use of a lower microwave frequency (915 MHz) to permit the achievement of very high microwave generation efficiency with commercially-available magnetrons. The AMET is a particularly attractive option for this class of missions because it provides specific impulse (~800 seconds) well beyond the reach of chemical propulsion, it provides high thrust per unit power to keep transit times acceptably short, and it permits the use of an easily-storable propellant (water) which is known to be available on both the Moon and Mars. ORBITEC staff has experience operating microwave electrothermal thrusters with water vapor as propellant. In Phase I, the AMET will be demonstrated with water vapor propellant to demonstrate feasibility, reaching TRL 4. In Phase II, a flight-like AMET will be developed and demonstrated and a design will be prepared for an entire AMET flight propulsion system, reaching TRL 6.