al elements that constitute a hierarchy of control, i.e. each element will operate on a specific level of control: Sensors and effectors operate at a very low level of control. A sensor detects some physical parameter of a resource and reports values to its resource control- ler. An effector affects some physical parameter of a resource, ordered to do SO by its resource controller. Partly as a result of orders from the process controller, resource controllers per- form state changes of resources. A resource state change involves activities from both effectors and sensors. Resource controllers may also perform some control functions autonomously, e.g. report spontaneous resource state changes to the process controller, or keep the resource in a steady state, which involves both detection and effectuation. The process controller has the overall responsibility for keeping the processing system in a consistent state and, through communication with resource control- lers, changing that state according to processing needs. Process controllers work with the highest degree of autonomy (system operators disregarded). We still do not have a model that comprises all types of elements that are associ- ated with processing systems. For example, from the previous model we can con- 1 Introduction to Systems Modelling clude that a process control system is meaningless, unless there is a resource system to control. The essential utilities are provided as some kind of M/E/I flow that are handled by the resource system, not by the process control system. Conse- quently, there must exist some users that are the sources for input flows, and the sinks for output flows of the resource system. Users can be humans, machines or both. There are more functional elements to deal with before we have a complete gen- eral model of processing systems. Although the system might be designed to act quite autonomously, there must exist process operators who plan and monitor the activities of the processing system, and who start and stop them. Operat