Life support systems (LSS) will be needed for all manned space missions and include such diverse applications as transportation vehicles, surface habitats, and rovers. Selecting an optimal LSS for a particular mission is the task of life support systems analysis, which must derive parametric values (e.g. power consumption, mass and volume) for the different possible LSS configurations. The task is labor intensive and time consuming, due to the many possible levels of life support system closure, the many available life support technologies, the equally large number of potential mission applications, and the complexity of system interactions. This paper describes TRIALSS (Tool for Rapid and Intelligent Advanced Life support system Selection & Sizing), a tool which makes use of modern software technology to assist in the LSS analysis process. TRIALSS has an icon-driven, user friendly interface which allows the user to select from several physical/chemical LSS technologies. It displays power, mass, and volume at system, subsystem, and assembly levels. A unique feature of TRIALSS is the ability to easily vary mission parameters including crew size, mission duration and location, power and thermal control system technologies, pressurized volume construction, and launch and transportation vehicle options. A powerful development environment, G2, allows the programmer to make rapid changes or additions and to incorporate rules to help the user avoid pitfalls. An example analysis for lunar applications shows the importance of mission context for system-level analysis and demonstrates the utility of TRIALSS in enabling such analyses.