Niklas Luhmann's sociological systems theory sees in an organisation neither a group of acting individuals nor a single corporative actor, but instead a social system. For Luhmann, a social system is a closed network of communication processes that recursively engender each other. This article introduces the reader to this particular way of understanding organisations. For this purpose, we will have to look at certain individual issues within Luhmann's theory. We will start with a few explanatory notes on Luhmann's theory of communication. Subsequently, key terms from the field of systems theory such as ‘system’, ‘medium/form’, ‘autopoiesis’ and ‘structure’ will be introduced. With the help of these notions, we will be able to clarify what Luhmann understands by a social system in general. For Luhmann, organisations are social systems of a particular type. In the last two sections, the specific characteristics of organisations as subsystems of the society will be outlined.