This paper focuses on the criteria for a comparison of legal concepts between different legal backgrounds. The traditional concept of equivalence must be revised, since absolute equivalence is, between national legal systems each with its own ethical principles and political priorities, no longer possible. A comparison on the grounds of a definition by intension becomes difficult because of the indeterminacy of legal concepts which are bound to moral values of a given society in a certain period. The 'tertium comparationis' for comparative terminography has to be found in the function of a concept within the framework of a specific legal solution, be it laws, court decisions, regulations or other legal provisions. A new comparative approach must be developed which does not aim at complete conceptual correspondence but at complete documentation of the national concepts. The ultimate goal of terminology work in law is to inform the user of the concepts used by national legal systems to control specific social circumstances.