The opposition procedure before the European Patent Office (EPO) is a post-grant, contentious, inter partes, administrative procedure intended to allow any European patent to be centrally opposed. European patents granted by the EPO under the European Patent Convention (EPC) may be opposed by any person from the public (no commercial or other interest whatsoever need be shown). This happens often when some prior art was not found during the grant procedure, but was only known by third parties. An opposition can only be based on a limited number of grounds, i.e. on the grounds that the subject-matter of the patent is not patentable, that the invention is insufficiently disclosed, or that the content of the patent extends beyond the content of the application as filed. The notice of opposition to a European patent must be filed in writing at the EPO (either at Munich, The Hague or Berlin), along with the payment of an opposition fee, within nine months from the publication of the mention of the grant of the patent in the European Patent Bulletin. Opposition Divisions of the EPO are responsible for the examination of oppositions. The opposition procedure may involve multiple opponents. According to the EPO, a European patent was once opposed by a record number of 27 opponents. Purpose of the opposition proceedings, extent of the opposition, and available grounds for opposition The purpose of the opposition proceedings is to give members of the public, such as competitors, the opportunity to challenge the validity of a granted European patent. No commercial or any other interest whatsoever need be shown. That is, the "opponent's motives are of no relevance for the purposes of the opposition procedure". Opponents may also challenge only some of the claims of the European patent (although this is in practice rather unusual), therefore limiting the extent of the opposition to the challenged claims. However, an opposition can only be based on a limited number of grounds (i.e., objections) specified in the European Patent Convention, namely the grounds listed in . An opposition can be based on the grounds that the subject-matter of the patent is not patentable (Article 100(a) EPC, for instance because the claimed invention is not new or not inventive), on the ground that the invention is insufficiently disclosed to allow a person skilled in the art to carry it out (Article 100(b) EPC), or on the ground that the content