This paper presents an introduction to process algebras. In the first part of the contribution we introduce a basic process algebra and give an informal interpretation of its operators. In the second part of the contribution we show how expressions of process algebra can be interpreted as transition systems. Using the robot example of this series the basic concepts are illuminated. We discuss a core element of process algebra, its possibility to compare observable behaviour of different processes. A short overview over different approaches within process algebra concludes the contribution.