Can anyone challenge a patent, or only someone being sued?

Anyone. Inter partes review and post-grant review can be filed by any person other than the patent owner, and ex parte reexamination can be requested by anyone, anonymously. A district-court challenge needs a case — either you are sued for infringement and plead invalidity as a defense, or you have a real, immediate dispute and file a declaratory-judgment action.

What is the difference between inter partes review and ex parte reexamination?

In an IPR you are a party: you file the petition, argue before three administrative judges, cross-examine experts, and get a final written decision — and you are estopped afterwards from raising grounds you raised or reasonably could have. In ex parte reexamination you file the request and step out: the Office re-examines the patent with only the patent owner participating. Reexamination costs a fraction of an IPR, carries no estoppel, and cannot be denied for the discretionary reasons IPRs increasingly are, but you have no say once it starts and the owner can amend the claims around your art.

How long does it take to invalidate a patent?

An IPR runs about eighteen months from petition to final written decision — up to six for the institution decision, then twelve for the trial. Ex parte reexamination has no statutory clock but typically takes one to two years. A district-court invalidity verdict arrives with the rest of the case, often two to three years after filing, and can be appealed.

What does it cost to invalidate a patent?

The Office fees for an IPR petition run into the tens of thousands of dollars before counsel and experts, and a contested IPR through final decision commonly costs several hundred thousand. Ex parte reexamination fees are a fraction of that. Litigating invalidity in district court is the most expensive route by far. The prior-art search that every route starts with is the smallest line item — $899 to $1,500 here — and it is the one that decides whether the rest is worth spending.

Can a patent be invalidated without prior art?

Yes, on some routes. District court and post-grant review allow non-prior-art grounds: patent-ineligible subject matter under §101, lack of written description or enablement under §112, and, in court, inequitable conduct or the on-sale bar. Inter partes review and ex parte reexamination do not — they are limited to anticipation and obviousness over patents and printed publications. In practice prior art is the ground that wins most often on every route, because it is the one the examiner was supposed to have found.

Guides

How to invalidate a patent: the four routes, and what art each one needs

Michael W. Smith, former USPTO patent examiner · September 13, 2026

A US patent is invalidated one of four ways: by a defense in district court, where any ground is available but the challenger must prove invalidity by clear and convincing evidence; by inter partes review at the Patent Office, limited to patents and printed publications but decided on a preponderance of the evidence; by post-grant review, any ground but only within nine months of grant; or by ex parte reexamination, which anyone can request at any time on printed art alone. Which route fits depends on where you stand, what art you have, and the calendar — but all four begin the same way, with prior art the examiner never saw.

The four routes at a glance

Route	Who	When	Grounds	Standard
District court (defense or declaratory judgment)	A defendant, or anyone with a live dispute	Any time the patent is asserted or threatened	Any: §102, §103, §101, §112, on-sale, inequitable conduct	Clear and convincing evidence (the patent is presumed valid)
Inter partes review (IPR)	Anyone but the owner; a sued defendant within one year of service	After nine months from grant (or after any PGR ends)	§102 and §103 only, on patents and printed publications only	Preponderance of the evidence
Post-grant review (PGR)	Anyone but the owner	Only within nine months of grant	Any ground of invalidity	Preponderance of the evidence
Ex parte reexamination	Anyone, including anonymously; the owner too	Any time during the patent's life	§102 and §103 on patents and printed publications, raising a substantial new question	The examiner decides, as in original examination

Route 1: district court

If you are sued, invalidity is a defense you plead alongside non-infringement, and it goes to the jury with the rest of the case. Every ground is open — anticipation, obviousness, ineligible subject matter, an inadequate specification, a sale before the critical date — and unlike the Office routes you can rely on prior use and prior sale, not only documents. The price of that breadth is the standard: an issued patent is presumed valid, and you have to prove otherwise by clear and convincing evidence, to a jury, after a Markman ruling on what the claims mean. Invalidity contentions are due early under most local patent rules, so the search has to be done in the first months of the case, not the last.

Route 2: inter partes review

The route most challengers choose. You petition the Patent Trial and Appeal Board with grounds under §102 or §103 based on patents and printed publications, the Board decides within about six months whether to institute, and if it does, a trial before three administrative patent judges ends in a final written decision about a year later. The standard is a preponderance — no presumption of validity — which is why IPRs kill claims that would survive a jury.

Three constraints shape it. A defendant must file within one year of being served. The petition must show a reasonable likelihood of prevailing on at least one claim, and the Board can decline to institute at its discretion — for art already considered during prosecution (§325(d)), for a parallel district-court case that is far along, and, since 2025, for other reasons that have made institution noticeably harder. And after a final decision the petitioner is estopped from raising, in court or in another petition, any ground it raised or reasonably could have raised. What an IPR needs is covered in how much prior art an IPR petition needs.

Route 3: post-grant review

The same trial procedure as an IPR, with two differences: any ground of invalidity is available, including §101 and §112, and the petition must be filed within nine months of the patent's grant. That window is why PGRs are rare — it closes before most patents are asserted — and why competitors who watch a rival's portfolio issue can use it when nobody else can. The estoppel is correspondingly broader.

Route 4: ex parte reexamination

The oldest and cheapest route. Anyone — a competitor, a defendant, an anonymous third party, the owner — can ask the Office to re-examine a patent in light of patents or printed publications that raise a substantial new question of patentability. If the Office agrees, an examiner re-examines the claims with the owner as the only participant; the requester has no further role. Claims can be confirmed, amended or cancelled, and the result is appealable by the owner only.

Reexamination has come back into use as IPR institution has become less predictable: there is no one-year bar, no estoppel, no discretionary denial for a parallel case, and the fee is a fraction of an IPR's. Its weaknesses are the mirror image — you cannot argue, you cannot cross-examine, and an owner who amends around your art comes out with claims that are narrower but strengthened.

What art each route needs

District court: anything, including products, prior use, sales and offers for sale — but it must be proven to a jury, so documentary art with a clean date still does most of the work.
IPR and reexamination: patents and printed publications only. A product manual, a dated web page or a conference paper qualifies as a printed publication if it was publicly accessible before the priority date, and you carry the burden of showing that it was.
PGR: anything, within nine months.
All four: art the examiner did not already consider. Art that was cited during prosecution and overcome is the weakest possible foundation — the Board can refuse an IPR on it outright, and a jury hears that the Office already looked at it.

The examiner's search record is in the file wrapper of every issued patent: the classes searched, the databases, the queries. The art that invalidates is usually in the examiner's own field, one classification or one decade away from where the search stopped. Reading that record is the first step of every invalidity search here, and the tools page reads it for any US patent, free.

Choosing

Sued, with strong printed-publication art and inside the one-year window: an IPR, filed early enough that the district court may stay the case. Sued, with the best art being a product or a prior sale: the court is the only route that can use it. Watching a competitor's patent issue: a PGR before nine months runs out. Wanting the patent weakened without a fight, cheaply, anonymously, or after the IPR window has closed: ex parte reexamination. In every case the decision is only as good as the art in hand, which is why the search comes before the strategy and not after it.

What this costs here

An invalidity search suitable for any of the four routes is $2,500$899 for a patent's independent claims or $5,000$1,500 for every asserted claim, flat, with references charted element-by-element, the prosecution record screened so you know which art is new to the Office, and the public-accessibility evidence for non-patent references attached.