The analysis of ECLSS requires a variety of software tools ranging from spreadsheets to optimization programs and detailed performance simulation codes. The application of software tools at system, subsystem, assembly and component level has in the past resulted in a multitude of different programs for support of ECLSS projects.By using an interface oriented modelling technique assembly, subsystem and system simulation can be per formed in a standard and transparent way. Utilizing the experience gained with a simulation program for fuel cell systems a simulation framework with extended capability was built. The software provides a number of component simulation modules which can be modified or replaced by user-defined modules. The component models can be used separately or combined to form open or closed loops, networks or complete ECLS systems.The modularity of the basic concept and the use of several numerical solution techniques is described.As an example, the simulation of a simplified ECLS subsystem, the related input data and the computed results are discussed. Areas of further development towards a knowledge based simulation tool are outlined.