Abstract: A computer resident tool for specifying and studying the functioning of complex systems by defining their functions in terms of components and connections. The components comprise transactions between the tool and the user, functional rules which are triggered by transactions and can create transactions, and information entities which are defined, modified and interrogated by functional rules and have dependency relationships with other information entities. The tool validates data both as it is entered by the user and globally after data entry is complete. The tool also produces lists of inconsistent system data and graphical representations of components and connections.