This doctoral thesis tries to outline the characteristics of legal concepts in general and their impact on terminography. It focuses on the specific features of legal concepts which justify a "Special Theory of Terminology" as proposed by Wüster et al. for legal terms. The research is based on continental Code Law systems.Concepts reflect the facts laid down by law. In the course of the administration of justice these rules must be applied to the concrete situations of single cases. The main function of legal concepts is to control the interaction of humans. A precise intensional definition of concepts would hinder the adaptation of the abstract rule to the single case in question, and leave no room for interpretation of laws and the adaptation of rules to new or changed social and moral environments. Furthermore, the extensional definition of a legal concept can change considerably by application of the analogy principle, i.e. extending the applicability of a rule to other cases not provided for in the law. Legal concepts can therefore not be described adequately by intensional or extensional definitions.Establishing equivalences between concepts belonging to two or more legal systems therefore requires not just a comparison of definitions but an analysis of their respective functions within the legal system. Equivalence in this sense does not mean intensional identity of concepts. The aim of comparative legal terminology should be to provide an insight into the purpose of single concepts (and their terms) within the framework of a rule and a system of laws. Similar function or comparable purpose of the concept within a particular segment of the whole system of laws are the criteria for establishing links between concepts of different legal systems.