THE NASA SPACE STATION holds remarkable similarity to the modern nuclear powered submarine. Both are required to provide a 90-day mission capability during which all aspects of human habitation must be accomodated. The requirements for environmental control, and failure tolerance of environmental control systems, are also quite similar. Although the resources, operating environments, and mission objectives for the submarine and space station are distinctly different, the problems faced by the environmental control and life support system (ECLSS) engineer require application of the same technologies and design principles.This paper addresses the resources and mission requirements specific to the two vehicles, and discusses their influence on the design of the ECLSS. The design constraints for individual ECLSS subsystems are discussed with the resultant impact on design approach selected for each vehicle.