A functional analysis was performed to identify life support functions and interrelationships required for manned space exploration. Methods were identified to provide each of these functions, ranging from resupply of consumables to totally regenerative processes. Specific mission characteristics and their effect on advanced life support requirements are outlined. A preliminary assessment is made as to which life support functions are critical for missions of various duration. Technologies which have been selected for Space Station Freedom and associated degrees of closure are discussed and areas for future work are suggested.