The US Space Shuttle on-orbit waste heat rejection is currently accomplished through a combination of radiators and a Flash Evaporator System (FES). Three of the FES units have been rebuilt to date owing to corrosion problems. In addition, the FES has experienced freeze-ups on-orbit.As part of NASA's Orbiter Upgrade Program, a Water Membrane Evaporator (WME) is being developed as a replacement for the FES. The WME will be installed on the Space Shuttle starting in the fall of 2001. It will use hydrophobic micropore membrane technology to passively control a water liquid/vapor interface. Waste heat that will be acquired from the Orbiter Freon-21 coolant loops will evaporate water at the interface. The water vapor will pass through the membrane and be vented to space. The WME program takes advantage of the recent advances in hydrophobic micropore membrane technology to provide a simpler and more robust heat rejection device than the current FES.In the present work, the WME concept is described in detail and its development program is discussed. The results of the WME hydrophobic membrane tests performed to date are presented and a snapshot of the WME preliminary design is provided.