AbstractIt is shown how the (extended) systems approach can be successfully used to design mathematical models in a highly systematic way. The systems approach supports interdisciplinarity in that it encourages participation of non-mathematicians in the model design process. The systems approach also leads to a clear documentation of a model.The basic principle explained in this paper is thatindicesof a model representelementsof an element set of a system, and thatvariablesandconstantsof a model representattributesof the elements.Multi-indexedvariables and constants refer to attributes of‘complex entities’(from extended systems terminology) which cover the information between the elements.