This paper discusses the capabilities of the ECOSIM software which is currently under development for the European Space Agency. This software is primarily intended to simulate Environmental Control and Life Support (ECLS) systems. It may be also customised to the simulation of passive and active thermal control systems, power systems, and more generally any system which can be described as a set of interconnected physical components. ECOSIM shall provide a powerful continuous simulation language, a graphical user interface and a library of ECLSS custom-made components.After a brief overview of the major aims of the development, the software is discussed from its main aspects, the language and its mathematical formalism, the graphical user interface, and the ECLS systems customisation for representing the physical, chemical, and biological processes of interest. Examples are provided aiming at enhancing the understanding of the critical aspects of the language and the user interface. The overall architectural design of the software is also discussed.ECOSIM version 1 -to be released in 1992- is described in depth: it will provide capabilities for simulating air management systems of the degree of complexity encountered in the Hermes and Columbus projects.