An opposition proceeding is an administrative process available under the patent and trademark law of many jurisdictions which allows third parties to formally challenge the validity of a pending patent application ("pre-grant opposition"), of a granted patent ("post-grant opposition"), or of a trademark.
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European Patent Organisation
In the context of the proceedings at the European Patent Office (EPO), third parties may dispute the validity of a granted European patent by filing a post-grant opposition under the European Patent Convention (EPC). The term for filing an opposition with the EPO is nine months from the publication of the mention of the grant of the European patent in the European Patent Bulletin.
United States
Under United States patent law, an opposition proceeding is called a reexamination. Post-grant review provisions of the new patent law may affect a potential patent infringement defendant’s strategies in filing a declaratory judgment action. Subsequent to the Leahy-Smith America Invents Act (2011), any third party can challenge the validity of an issued patent using either post-grant review under 35 U.S.C. § 321 or inter partes review under 35 U.S.C. § 311. Both proceedings became effective September 16, 2012.
Trademarks
In the case of trademarks, third parties may use opposition proceedings to "oppose" the acceptance of a trademark application after it has been accepted and published for opposition purposes. If an opposition is defeated the trademark will proceed to registration. Some jurisdictions operate a "post-grant" opposition system, whereby opposition is not possible until after registration (e.g. Japan).