A state transition diagram can be used to represent a state-based system. For example, a computer system can be represented by a state transition diagram which consists of a set of logical (not physical) states and transitions between those states. A state can be denoted by a node which has a description of the state within it (e.g. on/off). Many complex systems have thousands or even millions of possible states.

Time is not taken into consideration with changes of state. They are presumed to be instantaneous. In some cases, a transition does not actually result in a change of state. If for example, something that is switched off has a transition applied to it which attempts to switch it off again, nothing will happen as it is already in that state of being switched off.

Arcs or events (arrows) are labelled with a description of the event it represents. Events applied to a node can cause a change of state in that node. The first arc pointing towards the initial state is left unlabelled. State transition diagrams can be used to test safety critical systems and to represent how computer systems work at a low level.

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