Abstract
Understanding how a complex system works as a whole can be difficult because it requires blending information about structure and behavior into a coherent whole that can be understood without reference to details of how its parts are constructed, behave internally, or interact. The problem is doubly difficult for software systems, because we do not any good large-scale models of such systems to keep in the mind's eye. We have details in code files, low level diagrams of software details (for example, class inheritance hierarchies), and system views of hardware environments, but these are not enough. We suggest that models of whole systems that we can diagram and hold in the mind's eye are so important for human understanding of complex systems of all kinds that, if they do not exist, they must be invented. Use case maps are an example of a model invented for this purpose. While use case maps were invented to deal with the problems of understanding software systems, they are useful for complex systems of all kids.