Niklas Luhmann's theory of social systems is increasingly recognized today as one of the most interesting and fruitful approaches in the field of social sciences. This glossary serves as a working tool that can act both as an introduction to Luhmann's complex theory and as a means to facilitate the application of the terms offered by this theory. The format of the glossary is designed to allow a modular approach to the theory. The explanations for each term average three to four pages. Each term is accompanied by a series of bibliographic references. A system of cross-references between the terms has been supplemented by reading paths that indicate specific connections between the terms in relation to thematic fields—such as self-reference, social differentiation, constructivism, and so on.