Models are mechanisms for communication. This unit looks at what a model is and what the process of modelling is about. The techniques discussed here are applicable to a wide range of systems and have one thing in common: they are all commonly used diagramming techniques. The five techniques are: data flow diagrams, use case modelling, activity diagrams, entity–relationship diagrams and state machines. This study unit is just one of many that can be found on LearningSpace, part of OpenLearn, a collection of open educational resources from The Open University. Published in ePub 2.0.1 format, some feature such as audio, video and linked PDF are not supported by all ePub readers.