In this paper, we present LogMap a highly scalable ontology matching system with "built-in" reasoning and diagnosis capabilities. To the best of our knowledge, LogMap is the only matching system that can deal with semantically rich ontologies containing tens (and even hundreds) of thousands of classes. In contrast to most existing tools, LogMap also implements algorithms for "on the y" unsatisfiability detection and repair. Our experiments with the ontologies NCI, FMA and SNOMEDCT conrm that our system can efficiently match even the largest existing bio-medical ontologies. Furthermore, LogMap is able to produce a "clean" set of output mappings in many cases, in the sense that the ontology obtained by integrating LogMap's output mappings with the input ontologies is consistent and does not contain unsatisfiable classes.