A preliminary conceptual design of the Environmental Control and Life Support System (ECLSS), consisting of an initial and a growth version, has been developed for the proposed Space Station. This paper addresses the systems engineering approach taken in the course of planning, integrating, and developing the conceptual design. The approach includes defining system requirements and groundrules for the trade study, formulating possible options for cycle closure, establishing system-level mass balances, performing the trade-off study in search of an optimal degree of cycle closure, and constructing the conceptual design. The interdependency of the oxygen, water, and food cycles in the ECLSS is discussed and analysed with system-level mass balances. The trade-off study, which focuses on launch and resupply weight and volume requirements as well as life cycle costs of the system, is summarized. The study strongly suggests that a partially closed cycle ECLSS using state-of-the-art regenerative life support technology is the optimal option for cycle closure.