This introductory text provides a very brief overview of comparative research on multi-level systems. It includes national (federations, but also decentralized and regionalized states) and supranational (such as the European Union) multi-level systems. The text shows how multi-level systems can be categorized and discusses indices with which research tries to measure the degree of diversity. Some focal points of multi-level systems, as the distribution of competences, the decision-making processes, questions of legitimacy as well as reasons for dynamics in multi-level systems are discussed. At the end, the text gives a short overview of the texts on the topic of secession, which are included in the following chapter.