Burdens and Standards of Proof

Common situations.

• A defendant weighs whether to challenge a patent in court or at the Patent Office.
• A party that lost at the Board wonders which invalidity arguments it can still make in court.
• A patent owner facing a declaratory judgment suit asks who must prove what.

The standards at a glance.

Infringement.

The patent owner bears the burden of proving infringement by a preponderance of the evidence. The burden stays on the patent owner even when the accused infringer filed first, asking for a declaration of noninfringement (Medtronic, Inc. v. Mirowski Family Ventures, LLC, 571 U.S. 191 (2014)).

Validity in court.

An issued patent is presumed valid, and each claim is presumed valid independently of the others (35 U.S.C. § 282(a)). A challenger must prove invalidity by clear and convincing evidence. The standard does not drop when the challenger relies on prior art the examiner never saw, though the jury may be told it can consider that fact in deciding whether the burden is met (Microsoft Corp. v. i4i Ltd. P'ship, 564 U.S. 91 (2011)).

Obviousness is a legal conclusion resting on facts: the scope of the prior art, the differences between it and the claims, the level of ordinary skill, and objective evidence such as commercial success and long-felt need (Graham v. John Deere Co., 383 U.S. 1 (1966)).

Validity at the PTAB.

In inter partes review and post-grant review, the petitioner must prove unpatentability by a preponderance of the evidence (§§ 316(e), 326(e)), and the presumption of validity does not apply. The petitioner keeps the burden of persuasion throughout, including on whether a reference qualifies as prior art (Dynamic Drinkware, LLC v. Nat'l Graphics, Inc., 800 F.3d 1375 (Fed. Cir. 2015)). Since November 13, 2018, the Board construes claims under the same standard courts use (37 C.F.R. § 42.100(b)).

Estoppel. A petitioner that receives a final written decision cannot argue in court that a claim is invalid on any ground it raised or reasonably could have raised in the review (§ 315(e)(2); Cal. Inst. of Tech. v. Broadcom Ltd., 25 F.4th 976 (Fed. Cir. 2022)). The estoppel reaches only grounds based on patents and printed publications. A challenger can still rely on a product that was on sale or in public use, even if documents describing it were raised at the Board (Ingenico Inc. v. IOENGINE, LLC, No. 2023-1367 (Fed. Cir. May 7, 2025)). See PTAB Proceedings and Parallel Litigation.

Unenforceability.

An accused infringer claiming inequitable conduct must prove, by clear and convincing evidence, that the applicant withheld or misrepresented information with specific intent to deceive the Patent Office, and that the Office would not have allowed the claim but for the misconduct. Intent may be inferred, but only if deception is the single most reasonable inference (Therasense, Inc. v. Becton, Dickinson & Co., 649 F.3d 1276 (Fed. Cir. 2011) (en banc)).

Damages and marking.

The patent owner must prove the amount of damages by a preponderance. If the owner or its licensees sold unmarked products, damages run only from actual notice (§ 287(a)). The accused infringer must first identify the unmarked products it contends practice the patent; the patent owner then bears the burden of proving those products do not practice it or were marked (Arctic Cat Inc. v. Bombardier Recreational Prods. Inc., 876 F.3d 1350 (Fed. Cir. 2017)).

Willfulness and attorney's fees.

Enhanced damages for egregious infringement need proof only by a preponderance, and the judge has discretion over whether and how much to enhance (Halo Elecs., Inc. v. Pulse Elecs., Inc., 579 U.S. 93 (2016)). A party seeking fees in an exceptional case must also show entitlement by a preponderance (Octane Fitness, LLC v. ICON Health & Fitness, Inc., 572 U.S. 545 (2014)), and the trial court's ruling is reviewed for abuse of discretion (Highmark Inc. v. Allcare Health Mgmt. Sys., Inc., 572 U.S. 559 (2014)).

Claim construction.

The judge decides what the claims mean (Markman v. Westview Instruments, Inc., 517 U.S. 370 (1996)). On appeal, the Federal Circuit reviews the ultimate construction de novo but defers to the trial court's findings about extrinsic evidence such as expert testimony (Teva Pharms. USA, Inc. v. Sandoz, Inc., 574 U.S. 318 (2015)).

When a patent has been held invalid before.

A final judgment holding claims invalid prevents the patent owner from asserting those claims against anyone else, if the owner had a full and fair chance to litigate (Blonder-Tongue Labs., Inc. v. Univ. of Ill. Found., 402 U.S. 313 (1971)). A judgment upholding validity does not bind a new defendant, who may try again.

Texas notes.

• In the Eastern and Western Districts of Texas, juries decide most patent cases tried to verdict, so the clear-and-convincing instruction on validity often matters at trial.
• Defendants sued in Texas frequently file inter partes review petitions to use the lower PTAB standard. Whether the Board will institute review has become less predictable since 2025. See PTAB Proceedings and Parallel Litigation.

Sources.

• 35 U.S.C. §§ 282, 284, 285, 287, 315(e), 316(e), 326(e).
• 37 C.F.R. § 42.100(b).
• Medtronic, Inc. v. Mirowski Family Ventures, LLC, 571 U.S. 191 (2014).
• Microsoft Corp. v. i4i Ltd. P'ship, 564 U.S. 91 (2011).
• Graham v. John Deere Co., 383 U.S. 1 (1966).
• Dynamic Drinkware, LLC v. Nat'l Graphics, Inc., 800 F.3d 1375 (Fed. Cir. 2015).
• Cal. Inst. of Tech. v. Broadcom Ltd., 25 F.4th 976 (Fed. Cir. 2022).
• Ingenico Inc. v. IOENGINE, LLC, No. 2023-1367 (Fed. Cir. May 7, 2025).
• Therasense, Inc. v. Becton, Dickinson & Co., 649 F.3d 1276 (Fed. Cir. 2011) (en banc).
• Arctic Cat Inc. v. Bombardier Recreational Prods. Inc., 876 F.3d 1350 (Fed. Cir. 2017).
• Halo Elecs., Inc. v. Pulse Elecs., Inc., 579 U.S. 93 (2016).
• Octane Fitness, LLC v. ICON Health & Fitness, Inc., 572 U.S. 545 (2014).
• Highmark Inc. v. Allcare Health Mgmt. Sys., Inc., 572 U.S. 559 (2014).
• Markman v. Westview Instruments, Inc., 517 U.S. 370 (1996).
• Teva Pharms. USA, Inc. v. Sandoz, Inc., 574 U.S. 318 (2015).
• Blonder-Tongue Labs., Inc. v. Univ. of Ill. Found., 402 U.S. 313 (1971).

This guide is general information, not legal advice. Law checked through October 20, 2026. See the Disclaimer.