PTAB and Inter Partes Review (IPR) Explained

If you have been sued for patent infringement, or you are worried a competitor’s patent is blocking you, one of the most important strategic questions is whether you can attack the patent’s validity at the PTAB instead of — or alongside — fighting it out in court. Here is how these proceedings actually work.

What is the PTAB?

The Patent Trial and Appeal Board (PTAB) is an adjudicative body within the U.S. Patent and Trademark Office, created by the America Invents Act (AIA) of 2011. It is staffed by administrative patent judges (APJs) — technically trained lawyers, usually with science or engineering backgrounds — who sit in three-judge panels.

The PTAB does two main things:

• Hears appeals from examiners who reject patent applications during prosecution.
• Conducts AIA trials that let third parties challenge the validity of patents that have already issued.

It is that second function — the AIA trials — that reshaped patent litigation. Before 2012, invalidating a patent usually meant expensive, years-long litigation in federal district court; the older inter partes reexamination process existed but was slow and comparatively little used. The AIA created a faster, more trial-like administrative alternative, and the Supreme Court upheld its constitutionality in Oil States Energy Services v. Greene’s Energy Group (2018).

To see how these decisions play out across technologies, browse our patent case analysis archive.

What is inter partes review (IPR)?

Inter partes review is the workhorse of PTAB practice. It is a proceeding in which any person other than the patent owner asks the Board to cancel one or more claims of an issued patent as unpatentable.

The scope is deliberately limited:

• Grounds: only anticipation under 35 U.S.C. § 102 and obviousness under § 103. You cannot use an IPR to argue the invention is abstract or ineligible (§ 101) or that the specification is defective (§ 112).
• Evidence: only prior-art patents and printed publications — not product prior art, prior sales, or expert testimony about what was publicly used.
• Timing: a petition can be filed nine months after the patent grants (or after any PGR window closes), and there is no upper deadline except the one-year bar discussed below.

Because the vast majority of invalidity arguments rest on prior-art patents and publications, IPR handles the bulk of real-world validity challenges.

What is post-grant review (PGR), and how is it different?

Post-grant review is IPR’s broader but narrower-windowed cousin. It must be filed within nine months of a patent’s grant, and in exchange for that short window it allows a much wider range of challenges:

• § 101 — patent-eligible subject matter (e.g., abstract ideas, natural phenomena).
• § 112 — written description, enablement, and definiteness.
• §§ 102 and 103 — anticipation and obviousness, using any evidence, not just patents and printed publications.

PGR essentially lets a challenger raise almost any invalidity theory (best-mode challenges are excluded). The catch is that most defendants never see the patent as a threat within nine months of issuance, so the window usually closes before there is a reason to act. That is why IPRs vastly outnumber PGRs in practice. (A related proceeding, the transitional Covered Business Method program, sunset in 2020.)

What does the IPR timeline look like?

Speed is the entire point of a PTAB trial, and Congress wrote deadlines into the statute:

1. Petition filed. The challenger files a detailed petition — subject to a 14,000-word limit and usually accompanied by an expert declaration — identifying each claim, each ground, and each prior-art reference.
2. Patent owner preliminary response — due about three months later.
3. Institution decision. The Board must decide whether to institute within three months of the preliminary response — roughly six months from filing. It institutes only if the petitioner shows a reasonable likelihood it would prevail on at least one claim. Under SAS Institute v. Iancu (2018), institution is all-or-nothing: the Board must review every challenged claim and ground or none.
4. Trial phase. Discovery is limited, there is oral argument, and the final written decision must issue within one year of institution (extendable up to six months for good cause).

All in, a patent owner or challenger typically has a merits ruling in about 18 months — far faster than the three-to-five years district-court validity fights often take. Institution decisions are generally not appealable (Cuozzo Speed Technologies v. Lee, 2016), but the final written decision can be appealed to the U.S. Court of Appeals for the Federal Circuit.

Why is IPR such a powerful defense to infringement?

If you are accused of infringement, IPR gives you a second front — often a more favorable one. Several features tilt the field:

• Lower burden of proof. At the PTAB, a challenger only needs to prove invalidity by a preponderance of the evidence. In district court, an issued patent is presumed valid and can only be knocked out by clear and convincing evidence. That gap makes the same prior art meaningfully more likely to succeed at the PTAB.
• Expert decision-makers. APJs are patent specialists, not lay juries, which many defendants prefer for technical validity fights.
• Cost and speed. It is cheaper and faster than litigating validity to a jury (see below).
• Litigation stays. District courts frequently stay the parallel infringement case pending the IPR outcome, pausing an expensive lawsuit while the PTAB does the heavy lifting.
• Claim construction alignment. Since November 2018 the PTAB applies the same Phillips claim-construction standard used in federal court, so a patent owner can no longer play the two forums against each other by arguing for different claim meanings.

Because of this, filing an IPR is often the first move once a company is sued. We walk through the broader response playbook in what to do when someone is infringing your patent — which applies equally, in reverse, to a company on the receiving end of an assertion.

What is the one-year time bar and discretionary denial?

Two traps catch unwary petitioners.

The one-year time bar (§ 315(b)). If you have been served with a complaint for infringing the patent, you must file your IPR petition within one year of that service. Miss it, and you are barred from filing at all. This forces defendants to decide fast.

Discretionary denial. Even a strong petition can be turned away. Under § 314(a), the Board may deny institution when a parallel district-court case is far along — the line of decisions known as the NHK-Fintiv factors, which weigh trial dates, investment, and overlap. USPTO policy on discretionary denial has swung repeatedly through Director memoranda in recent years, so its scope in 2026 depends on current guidance and should be checked with counsel before you rely on it.

How much does an IPR cost?

There are two cost buckets.

Government fees. The USPTO charges an IPR request fee plus a post-institution fee, together running into the tens of thousands of dollars (with add-ons for challenging large numbers of claims). These fees have risen with periodic USPTO fee adjustments, so confirm the current schedule before budgeting.

Legal fees. This is where the real money is. According to AIPLA economic surveys, all-in IPR costs commonly run roughly $300,000 to $600,000 through a final written decision. That is a serious investment — but compare it to litigating patent validity to a jury in district court, which frequently costs several million dollars per side. For an accused infringer, IPR is usually the cheaper path to the same goal: killing the patent.

What is IPR estoppel?

The trade-off for that cheaper shot is estoppel, and it is severe. Under § 315(e), once the Board issues a final written decision, the petitioner is barred from later arguing — in the USPTO, in a district court, or at the International Trade Commission — any ground it “raised or reasonably could have raised” during the IPR.

Practically, this means:

• You generally get one bite at invalidating the patent on prior-art patents and publications.
• Your petition must front-load your best references, because you cannot hold some back for court.
• Estoppel does not reach grounds an IPR can’t consider (like § 101 or product prior art), so some defenses survive for court.

This is why petition strategy is a specialized craft — and why it should be coordinated with your overall litigation and business plan, including any patent licensing or monetization angle. A related decision is whether to run an IPR at all, or instead protect an innovation outside the patent system entirely, a trade-off we cover in patent vs. trade secret.

How does IPR fit into fighting patent trolls?

IPR was designed, in large part, as a check on weak patents — including those asserted by non-practicing entities. A patent-assertion entity that sues dozens of companies on a shaky patent faces the risk that one defendant files an IPR, wins a final written decision cancelling the claims, and destroys the patent for everyone, because cancellation is in rem against the patent itself.

That leverage changes settlement math: a credible IPR threat can turn a nuisance demand into a walk-away. We cover the broader dynamic — and how to evaluate a demand letter — in what patent trolls are and how to respond. IPR is one of the sharpest tools in that fight, sitting alongside the enforcement and defense strategies in our patent enforcement and monetization hub.

The bottom line

The PTAB gave patent disputes a faster, cheaper forum for one specific question: is this patent actually valid? Inter partes review — available nine months after grant, limited to §§ 102/103 grounds on patents and printed publications — is the primary tool, offering a lower burden of proof, expert judges, and an ~18-month path to a decision. Post-grant review covers broader grounds but only in the first nine months. The price of that efficient second shot is § 315(e) estoppel: you must bring your best prior art and get it right the first time. For any company facing an infringement threat, whether to file an IPR — and when, given the one-year bar — is among the most consequential early decisions in the case.

This guide is general education, not legal advice, and does not create an attorney-client relationship. PTAB strategy, estoppel, and filing deadlines turn on the specific facts and posture of your dispute — consult a patent litigator licensed in your jurisdiction before acting.