The Spacelab Environmental Control/Life Support Subsystem (ECLS) provides an earth-like “shirtsleeve” environment in the Spacelab cabin compatible with the atmosphere of the Orbiter crew compartment. This paper describes the major ECLS functions with particular respect to the current Flight Unit hardware status. It also includes recent design modifications, improvements, and adaptations resulting from Spacelab qualification and integration. ECLS potential atmospheric interaction with the Orbiter ECS is presented as well. In the course of extensive performance testing, the Spacelab ECLS design has proven its capability to meet specified performance and to provide adequate operational support.