The Space Station requirements are divided into eleven systems. One of these systems, the Environmental Control/Life Support System (ECLSS) is further divided into seven functional categories as follows: Atmosphere Revitalization System, Atmosphere Pressure and Composition Control System, Module Temperature and Humidity Control System, Water Management System, Waste Management System, EVA Support and Safe Haven. The paper reviews the requirements for ECLSS in terms of the initial and growth operational capabilities of the Reference Space Station architecture.The paper reviews some of the results of a systems engineering study under way. Both regenerative and nonregenerative ECLSS techniques are reviewed. A design for all of the primary and backup technologies was established so that accurate trade studies could be performed. Each technology design started at a common interface condition for competing technologies. The designs were defined by developing a mechanical schematic and control requirements. The actuators, sensors, control/monitor requirements, weight, power and volume of each design were developed from these designs. The design aspect of the technology maturity and reliability indicated the impact that each design would have on the IOC Space Station. The reliability was also evaluated in terms of impact upon the on-orbit spares and resupply requirements. The control/monitor instrumentation was evaluated from a capabilities, location and genericness standpoint within each module.Individual subsystems, assemblies, units and instruments are discussed in terms of requirements, sizes and locations within the modules of a Space Station (for the CDG, Planar and Reference Space Station Configurations). The synergism between the ECLSS and other systems, that is, the commonality of the water, oxygen, hydrogen utility lines with the Electric Power System and the Reaction Control System are discussed.Phase B of the Space Station Design will critically evaluate the ECLSS alternatives and promote a preliminary design. This paper discusses a powerful tool to be used in this evaluation-a wide range data base developed from a common basis. The utilization of this data base will ensure accurate trade studies and confidence in the ECLSS design.