A key problem in case-based reasoning is the representation, organization and maintenance of case libraries. While current approaches rely on heuristic and psychologically inspired formalisms, terminological logics have emerged as a powerful representation formalismwith clearly de ned formalsemantics. This paper demonstrates how the indexing of case libraries can be grounded on terminological logics by using them as a kind of query language to the case library. Indices of cases are represented as concepts in a terminological logic. They are automatically constructed from the symbolic representation of cases with the help of a well-de ned abstraction process. The retrieval of cases from the library is grounded on concept classi cation. The theoretical approach provides the formal foundation for the fully implemented case-based planning system MRL. The use of terminological logics allows formal proof of properties like the correctness, completeness and e ciency of the retrieval algorith...