We are living in an information society. For us it is normal to access relevant information almost immediately. In our world, information systems play an important role in our private as well as in our professional lives. When selecting or developing such systems, especially complex ones, we need to understand and model the requirements on these systems. This paper deals with the modeling of complex information systems. We show which requirements concepts could be modeled, but also argue that it is not necessary to model all concepts. We show empirical studies that make us believe that further empirical research is needed in order to know which requirements concepts are most relevant. Current challenges as well as future challenges with regard to information system modeling are outlined.
