Niklas Luhmann never self-identified as a relational sociologist. His is a theory of social systems. Some relational sociologists tend to assume that the concept of system can only be deterministic; others in the field have nonetheless taken an interest of Luhmann's theory. Luhmann brings in numerous concepts—autopoiesis, functional differentiation, operational closure, environment, complexity, self-reference, communication and re-entry—that redefine social systems as ongoing, dynamic, self-organizing processes. As a result, his theory aligns very well with the project or idea of a relational sociology. To defend this claim, the chapter presents an overview of Luhmann's systems theory. Next it sets up different criteria for defining what relational sociology is or could be, and it evaluates Luhmann's theory in light of these criteria. Lastly, the chapter discusses the works of four relational sociologists who have already engaged Luhmann's ideas: Stephan Fuchs, Harrison White, Jan Fuhse and Pierpaolo Donati.