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.

Patent opposition proceedings are administrative mechanisms within the patent law framework that allow third parties to formally challenge the validity of a pending patent application (pre-grant opposition) or a granted patent (post-grant opposition).^[1][2] These opposition proceedings serve as a crucial check within the patent system, ensuring that patents are granted only for inventions that genuinely meet all legal requirements, thereby maintaining the integrity and quality of the patent landscape.^[1]

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).^{[citation needed]}

In Canada, any third party can file a statement of opposition to stop a trademark from being registered for at least one of the reasons set out in the Trademarks Act and Trademarks Regulations.^[3] A statement of opposition can only be filed during the two-month period after a trademark application is approved by the Canadian Intellectual Property Office (CIPO) and advertised in the Trademarks Journal.^[4] The trademark applicant and opponent then submit pleadings, evidence and arguments to the Trademarks Opposition Board (an administrative body within CIPO), which hears and makes decisions in opposition proceedings.^[3][4][5] The Board can either refuse the trademark application (in whole or in part) or reject the opposition.^[4] This decision can be appealed to the Federal Court of Canada by both the trademark applicant and opponent.^[3][4]

• Patent watch
• 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")