After a short presentation of the different functions of the Environmental Control and Life Support System (ECLSS), the paper shows the driving role of the atmosphere delivery and pressure control subsystem.sThe main physiological requirements of environmental control are presented: oxygen delivery, total pressure and total pressure variations, carbon dioxide concentration. The acceptable limits of the different parameters are discussed and a comparison is made with the operational requirements of existing space vehicles. From this analysis a selection is made for nominal, degraded and emergency modes.A safety philosophy is presented and application is made to different emergency situations. For emergency situations, the proposed solution is intermediate between the Orbiter choice and the Soyouz approach.A mathematical model of the vehicle's partial pressures is presented. This computer program is used to optimize the pressure management in the emergency modes and in transient conditions like EVA prebreathing and airlock operation.