A new software tool, MELISSA, has been developed for the simulation of life-support systems and other network-type subsystems. MELISSA features an intuitive graphical modeling environment and interactive simulation execution. Applications of MELISSA range from the analysis and validation of new ECLSS designs, to parametric optimization studies, to failure mode effects and criticality analysis of life-support systems. Additionally, MELISSA can be employed for training ECLSS developers and users, and as a teaching tool for lectures and seminars on systems design. As a demonstration, an ECLSS similar to the one of the International Space Station has been modeled and simulated.