NASA's future missions to explore the solar system will be long-duration missions and could last years at a time. Human life support systems required for these missions must operate with very high reliability for long periods of time and must be highly regenerative, requiring minimum resupply. Such life support systems will make use of combining higher plants, microorganisms, and physicochemical processes to recycle air and water, process wastes, and produce food. Development of regenerative life support systems will be a pivotal capability for missions to the moon and Mars. One key step in the development process for these systems is the establishment of a human-rated test facility specifically tailored for evaluation of closed, regenerative life support systems--one in which long-duration testing can take place involving human test crews. The requirements for such a facility, the Closed Ecological Life Support Systems Human-Rated Test Facility (CELSS HRTF), are currently being defined and developed at NASA's Johnson Space Center, and long-lead construction projects relating to this facility have already commenced. This paper will provide an overview of the CELSS HRTF project, summarizing the early efforts to define project objectives, develop a reference mission scenario, and define system and facility requirements.