The design and development of crew emergency response systems, particularly to provide an unplanned emergency return to earth, requires an understanding of crew performance challenges in space. The combined effects of psychological and physiological adaptation during long duration missions will have a significant effect on crew performance in the unpredictable and potentially life threatening conditions of an emergency return to earth. It is therefore important that the systems to be developed for emergency egress address these challenges through an integrated program to produce optimum productivity and safety in times of utmost stress. Fundamental to the success of the CRV is the Environmental Control and Life Support System (ECLSS), which provides the necessary conditions for the crew to survive their return mission in a shirt-sleeve environment. This paper will discuss the many issues in the design of an ECLSS system for CRV and place it in the context of the human performance challenges of the mission.