What is prior art in patent law?

Prior art is the collective body of public knowledge that predates a patent applicant's effective filing date. Every patent claim is measured against this record: if the claimed invention was already disclosed to the public, the law will not grant an exclusive right to it. Prior art is the operative standard for both novelty and non-obviousness, the two core conditions for patentability.

What counts as prior art

Under AIA 35 U.S.C. § 102(a)(1), a claimed invention is unpatentable if, before the effective filing date, it was patented, described in a printed publication, in public use, on sale, or otherwise available to the public.

Printed publication is interpreted broadly. The category reaches patents, journal articles, conference posters, technical manuals, internet postings, and any document accessible to the public in any medium; a reference qualifies as of the date it became publicly accessible, not the date it was formally published or issued.

Public use under the AIA carries no geographic restriction. A public use of the claimed invention anywhere in the world, not only in the United States, constitutes prior art, which is a significant expansion from the pre-AIA framework that limited this category to domestic acts.

On sale catches more than arm's-length commercial transactions. A sale or offer for sale is prior art even when it is made under a confidentiality agreement, because the confidentiality does not shelter the commercial act from the statutory bar.

Otherwise available to the public is a catch-all for disclosures that do not fit neatly into the enumerated categories, for example an oral presentation at a public conference, a product demonstrated at a trade show, or an unpublished thesis shelved in a university library. The inquiry is whether the information was accessible to interested members of the public.

The § 102(a)(2) category: earlier-filed applications

Beyond the public-disclosure categories, a U.S. patent or published patent application naming a different inventor and effectively filed before the applicant's effective filing date also constitutes prior art under § 102(a)(2), even if the competing application had not yet published when the applicant filed. This provision catches filings that raced to the USPTO before yours but were not yet visible in the public record.

The effective filing date

Prior art is measured against the effective filing date, not the date of invention, and not necessarily the filing date printed on the application. The effective filing date is the earlier of the actual filing date or the filing date of the earliest application in the priority chain (provisional, nonprovisional, international, or foreign) to which the applicant has properly claimed priority or benefit under 35 U.S.C. §§ 119, 120, 121, 365, or 386.

A valid priority claim therefore expands the prior-art cutoff backward. A provisional application filed before a competitor's public disclosure can place that disclosure outside the prior art window, but only if the priority chain is formally intact and the subject matter being claimed was adequately supported in the earlier application. A disclosure gap in the chain breaks the priority benefit for that subject matter.

Exceptions: what is not prior art

Not every earlier disclosure automatically bars patentability. The AIA created two categories of exceptions.

Grace period (§ 102(b)(1)). Disclosures made by the inventor, or by someone who obtained the subject matter directly or indirectly from the inventor, within the statutory period before the effective filing date are excepted and do not constitute prior art. This means an inventor who publishes a paper, presents at a conference, or launches a product may still obtain a patent, provided the application is filed within the window set by § 102(b)(1).

The grace period also shelters against a specific intervening-disclosure trap: if the inventor already publicly disclosed the subject matter, a subsequent third-party disclosure of that same subject matter is also excepted, because the third party's disclosure cannot constitute independent prior art when the inventor was first to the public.

Common ownership (§ 102(b)(2)). A disclosure in another patent or published application does not constitute § 102(a)(2) prior art if the relevant patent and the applicant's invention were commonly owned, or subject to a common obligation to assign to the same entity, as of the effective filing date. This exception protects large research institutions and companies where multiple groups file overlapping applications and might otherwise create internal prior art problems.

How prior art is used against a claim

Prior art plays two distinct roles in examination and litigation. Both stem from separate statutory standards that impose independent tests.

Novelty (§ 102). A single prior art reference that discloses every element of the claimed invention, arranged in the same way, anticipates the claim and defeats it outright. Every element must be present in the one reference; combining two references is not permitted for a novelty rejection. Even a single missing element from the reference means the claim is not anticipated (though it may still be obvious).

Non-obviousness (§ 103). Under § 103, a claimed invention is unpatentable if the differences between the claimed invention and the prior art would have been obvious to a person of ordinary skill in the pertinent art at the effective filing date. Unlike novelty, the obviousness analysis can draw on combinations of multiple references. An examiner may argue that it would have been obvious to a skilled artisan to combine reference A with reference B to arrive at the claimed invention, motivated by a suggestion in the art to do so.

A prosecution strategy often must address both: distinguish the closest reference for novelty, then demonstrate why the gap between the claim and the prior art combination is not obvious.

The duty to disclose known prior art

Knowing about prior art creates a disclosure obligation, not just a litigation risk. Every person substantively involved in preparing or prosecuting a patent application has a duty of candor and good faith that requires disclosing all information known to be material to patentability. The mechanism is the Information Disclosure Statement (IDS), filed with the USPTO.

Filing is subject to timing requirements. An IDS submitted within three months of the national application filing date, or before the first office action on the merits, is considered without additional fee. Disclosure after those early windows requires either a fee or a statement of compliance, depending on the stage of prosecution.

A patent procured through intentional failure to disclose material prior art can be held unenforceable for inequitable conduct in litigation, an absolute defense to infringement, not merely a reduction in damages. The standard requires showing both materiality and intent.

Prior art in post-grant challenges

Prior art does not stop being relevant once a patent issues.

Inter partes review (IPR). In an IPR, a petitioner may challenge issued claims only on the basis of prior art consisting of patents or printed publications, on grounds of novelty or non-obviousness. This is a meaningful constraint: physical product prior art, trade-secret prior uses, and non-documentary public disclosures cannot be introduced in an IPR, even where they would defeat novelty at trial. Practitioners choosing between a declaratory judgment action and an IPR petition need to account for this limitation.

Post-grant review (PGR). A PGR allows challenges on any ground of invalidity cognizable under § 282(b), which is a broader arsenal than the patent-and-printed-publication limit that governs IPR. PGR must be petitioned within the statutory window after patent grant; after that period closes, IPR is the only remaining USPTO administrative route for prior-art challenges.

Practical notes for practitioners

• Search before you draft claims. A thorough prior art search tells you where to differentiate, what language to avoid, and whether the invention is patentable at all before investing in a full application.
• Date public accessibility, not creation. A reference becomes prior art when it was accessible to an interested member of the public. A conference poster on display is prior art from the date of the conference, even if later published in a journal months afterward.
• Trace the priority chain carefully. Each application in the chain must adequately support the specific subject matter being claimed. An adequate disclosure in the provisional for some claims may be insufficient for others; confirm coverage before relying on the earlier filing date.
• Disclose material references promptly. Early IDS submission avoids fees and reduces inequitable conduct exposure. Do not wait until prosecution is advanced to submit references identified at filing.
• Assess the grace period before counseling clients who have already disclosed. Map each prior disclosure, confirm it falls within the statutory window, identify any third-party disclosures made between the inventor's disclosure and the filing date, and determine whether the identical subject matter exception covers them.
• Choose the right challenge venue. If a key invalidity argument turns on prior public use or a trade-secret prior use, an IPR cannot deploy it; a district court invalidity defense can.