Of the possible Life Support Systems evaluation criteria, the criterion of "integral reliability" is proposed. This criterion incorporates three main indices: reliability, mass, and quality of life. It is possible to interrelate these indices only if the space mission is considered as a whole. It is shown that there must exist a LSS mass optimum with respect to mission reliability. The specific form of "integral reliability" expression and the number of terms depend on the mission scenario. This work considers different LSS for orbital station, Lunar base, and Mars mission scenarios.