ECOSIM is a software tool for continuous simulation of systems which makes possible a combined representation of different physical aspects of a system, such as fluid flows, chemical reactions, electrical phenomena, and analogue and digital controls, in a single model.ECOSIM is a true modular simulation tool. The user can easily develop re-usable submodels and libraries of parametric components in the ECOSIM language and by taking advantage of the abstractions provided by the program. It is also possible to call FORTRAN or C subroutines.Mathematical modelling is based on an underlying differential algebraic equation solver, which overcomes the common drawbacks of simulators based on ordinary differential equation solvers. This permits very rich equation sets to be employed in the model which in turn opens the door to multi-disciplinary simulation.These capabilities make ECOSIM a very powerful tool for the simulation of Environmental Control and Life Support Systems (ECLSSs). New capabilities like inheritance - to facilitate the development of libraries of re-usable submodels - are currently being implemented to enhance further the power of the component libraries.