Csl-lean: A theorem-prover for the logic of comparative concept similarity. The logic CSLCSL of the comparative concept similarity has been introduced by Sheremet, Tishkovsky, Wolter and Zakharyaschev to capture a form of qualitative similarity comparison between concepts and/or objects. In this logic we can formulate assertions of the form “objects A are more similar to B than to C”. This kind of assertions can be added to an ontology to express qualitative comparisons between concepts. In this work we present CSL-lean, the first theorem-prover for this logic. It is a direct Prolog implementation of a tableaux-based decision procedure recently proposed for this logic. The Prolog program is inspired by the lean-methodology. CSL-lean also contains a graphical interface written in Java and it is available for free download at http://www.di.unito.it/ pozzato/csllean/.