Under a presidential directive, the United States has begun preparations to return to the moon. The purpose of this new initiative is not to repeat what the Apollo missions accomplished earlier, but to build on these accomplishments. President Bush has called for a long range continuing commitment to expand human presence beyond Earth orbit. This is a program that will span decades. It will establish a permanent station on the Moon and send a manned mission to Mars. The challenge is to develop the systems and technologies necessary to travel to the Moon and Mars and to provide an environment in which the crew can safely live and work in both of these new worlds. Perhaps the most critical technology to the success of these advanced missions is that related to the basic environmental control and life support system. Initially, much of the required technology will stem from NASA's experience on Space Station Freedom, although the extended duration of the missions will force the technology to evolve in a way that satisfies a new set of requirements. This is clearly the case in the area of the life support systems. The presence of limited gravity will permit some design flexibility not found on the space station, however the longer intervals between resupply will create new problems in waste management and the availability of consumables.This paper discusses the options available for regenerative environmental control and life support systems (ECLSS) design, and how computer modeling might help determine the advantages and disadvantages of the different system configurations.