Inter partes review is an administrative trial proceeding, created by the America Invents Act and codified at 35 U.S.C. §§ 311–319, through which a third party can ask the U.S. Patent and Trademark Office to cancel claims of an already-issued patent. The proceeding is conducted by the Patent Trial and Appeal Board (PTAB), a panel of administrative patent judges. The statute opens the door narrowly: § 311(a) provides that "a person who is not the owner of a patent may file with the Office a petition to institute an inter partes review of the patent." The patent owner cannot use IPR against its own patent.

The grounds are limited. Section 311(b) confines an IPR to anticipation and obviousness challenges built on documentary prior art. A petitioner cannot raise patent-eligibility under § 101, indefiniteness or written-description defects under § 112, or prior public uses and sales — those arguments live in district court or in other proceedings, not in an IPR.

"A petitioner in an inter partes review may request to cancel as unpatentable 1 or more claims of a patent only on a ground that could be raised under section 102 or 103 and only on the basis of prior art consisting of patents or printed publications."— 35 U.S.C. § 311(b), source

The institution threshold

Filing a petition does not start a trial. The Board first decides whether to institute, and the statutory threshold is set by 35 U.S.C. § 314(a): the Director "may not authorize an inter partes review to be instituted unless" the petition and any patent-owner preliminary response show "that there is a reasonable likelihood that the petitioner would prevail with respect to at least 1 of the claims challenged in the petition." The phrase "may not" sets a floor, not a ceiling — the Supreme Court has read the institution decision as discretionary, and the Board can decline to institute even when the reasonable-likelihood standard is met.

That discretion has become a recurring battleground. The Board applies discretionary-denial factors developed in its precedential decisions to weigh the burden of parallel district-court litigation and other considerations. Whether and when the Board exercises that discretion is set by USPTO guidance rather than the statute, and it shifts over time; the statutory anchor, however, remains the reasonable-likelihood threshold in § 314(a).

By statute the institution decision itself is largely insulated from appeal. Section 314(d) provides that the determination whether to institute "shall be final and nonappealable." The Supreme Court applied that bar in Cuozzo Speed Technologies, LLC v. Lee (2016) and Thryv, Inc. v. Click-To-Call Technologies, LP (2020), and the Federal Circuit continues to enforce it against arguments that merely repackage an institution complaint as a defect in the final decision.

From institution to final written decision

Once instituted, an IPR proceeds to a trial on the written record with limited discovery, a patent-owner response, an optional motion to amend the claims, and an oral hearing. The statute directs the Board to issue a final written decision on the patentability of the challenged claims, and § 316(a)(11) sets a one-year deadline from institution — extendable by up to six months for good cause — for that decision. A party dissatisfied with the final written decision may appeal to the Federal Circuit under § 319.

An IPR also carries estoppel consequences. Under § 315(e), a petitioner who obtains a final written decision is estopped from later asserting, in the USPTO or in district court and ITC proceedings, any ground that it raised or reasonably could have raised during the IPR. That estoppel, combined with the prior-art-only scope and the lower preponderance burden the Board applies, is why IPR has become the central forum for prior-art validity disputes that run parallel to infringement litigation.

Who can file and when

The statute also imposes timing and standing limits that shape IPR strategy. Under § 315(b), a petitioner served with a complaint alleging infringement of the patent generally must file its IPR petition within one year of service, or the petition is time-barred. That one-year bar is the reason an accused infringer that wants to use IPR moves quickly after suit is filed. Section 315(a) separately bars an IPR if the petitioner had already filed a civil action challenging the validity of the patent before petitioning.

The burden and standard of proof in an IPR differ from district court. In an instituted IPR the petitioner must prove unpatentability by a preponderance of the evidence, the lower civil standard, whereas an issued patent in district court is presumed valid and a challenger must prove invalidity by clear and convincing evidence. That difference in burden is one reason a prior-art defense can succeed in an IPR even when the same art might not carry the day before a jury, and it is part of why IPR became a favored forum for prior-art challenges.

Amendment is available but constrained. Section 316(d) permits a patent owner to file one motion to amend to cancel a challenged claim or propose a reasonable number of substitute claims, though the substitute claims cannot enlarge the scope of the original claims or introduce new subject matter. In practice, motions to amend are filed in a minority of proceedings, so most instituted IPRs are resolved on the patentability of the claims as originally issued. The combination of the prior-art-only scope, the preponderance standard, the one-year time bar, and statutory estoppel under § 315(e) defines the strategic shape of the IPR system.