A quantitative comparison of the merits/demerits of storage vs. regeneration of essential supplies for long duration extraterrestrial manned missions requires detailed life support systems analyses. This is the first part of a series of papers highlighting the importance of mission-driven system definition and assessment for extraterrestrial human life support. In this part, rationale and methodology for adopting the systems approachis discussed. A top-down hierarchical structurefor conceptual modeling of life support systems is also presented. The necessity for validation of all data utilized in any system assessment study is briefly discussed.