An opposition proceeding is an administrative process available under the patent and trademark law of many jurisdictions which allows third parties to formally dispute the validity of a pending patent application ("pre-grant opposition"), of a granted patent ("post-grant opposition") or of a trademark.
In the context of the proceedings at the European Patent Office, third parties may dispute the validity of a granted European patent by filing a post-grant opposition under the European Patent Convention (EPC).
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. Under the new Leahy-Smith America Invents Act, 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 are effective September 16, 2012. 
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).
- Patent watch
- Prior art
- Public participation in patent examination (including a discussion on the possibility, in some jurisdictions, for third parties to file observations during the examination of a patent application, see sub-section "Observations by third parties")