Other Defenses and Limits on Recovery

Common situations.

• A reseller is sued over products it bought from the patent owner's licensee.
• A manufacturer has used an internal process for years and is now accused of infringing a newly issued patent.
• A patent owner waited many years before suing.
• A government contractor is accused over products made for a federal agency.
• The inventor who assigned the patent now competes with its buyer.

License.

A license, express or implied, is a complete defense to the extent of its scope. Licenses granted by one co-owner bind the others (35 U.S.C. § 262). Implied licenses can arise from conduct, from a sale of a component with no noninfringing use, or from earlier dealings. The accused infringer bears the burden of proving a license.

Exhaustion (first sale).

An authorized sale of a patented item ends the patent owner's rights in that item. The buyer and later owners may use and resell it (Quanta Computer, Inc. v. LG Elecs., Inc., 553 U.S. 617 (2008)). Exhaustion applies even when the sale carries restrictions on reuse or resale, and even when the sale took place abroad. Those restrictions may be enforced as contract terms, but not through a patent suit (Impression Prods., Inc. v. Lexmark Int'l, Inc., 581 U.S. 360 (2017)). Exhaustion does not let a buyer make a new copy of the item.

Prior commercial use.

A party that commercially used the invention in the United States, in good faith, at least one year before the earlier of the patent's effective filing date or the patent owner's public disclosure has a defense (§ 273(a)). The defense:

• must be proved by clear and convincing evidence (§ 273(b));
• is personal and transfers only with the whole business it relates to (§ 273(e)(1));
• covers only the sites where the use took place before the critical date, when the business is transferred;
• does not apply against most patents owned by universities (§ 273(e)(5)).

This defense matters most for manufacturing processes kept as trade secrets. See Patent, Trade Secret, or Both?

Delay by the patent owner.

• Damages limit. No damages are recoverable for infringement more than six years before the complaint or counterclaim (§ 286).
• Laches. Laches cannot bar damages within that six-year window (SCA Hygiene Prods. Aktiebolag v. First Quality Baby Prods., LLC, 580 U.S. 328 (2017)). It may still affect equitable relief such as an injunction.
• Equitable estoppel. A patent owner whose misleading conduct, such as long silence after threatening suit, led the accused infringer to rely on it to its detriment may be barred entirely (A.C. Aukerman Co. v. R.L. Chaides Constr. Co., 960 F.2d 1020 (Fed. Cir. 1992) (en banc)).
• Prosecution laches. A patent can be unenforceable if the applicant caused an unreasonable and unexplained delay in prosecution that prejudiced others (Hyatt v. Hirshfeld, 998 F.3d 1347 (Fed. Cir. 2021); Hyatt v. Stewart, 148 F.4th 1376 (Fed. Cir. 2025), cert. denied, No. 25-1049 (U.S. June 29, 2026)). The defense requires proof of prejudice. In 2025 the Federal Circuit reversed a finding of prosecution laches because the accused infringer did not show it invested in the feature before the patent owner's application published (Google LLC v. Sonos, Inc., No. 2024-1097 (Fed. Cir. Aug. 28, 2025) (nonprecedential)).

Marking and notice.

If the patent owner or its licensees sell products covered by the patent without marking them, damages run only from the date the accused infringer received actual notice of infringement, such as a specific charge or the filing of suit (§ 287(a)). Virtual marking on a website listed on the product counts. See Burdens and Standards of Proof.

Unenforceability.

• Inequitable conduct. Deceiving the Patent Office renders the entire patent unenforceable. See Burdens and Standards of Proof for the Therasense standard.
• Unclean hands. Litigation or business misconduct directly related to the patent can bar enforcement (Gilead Scis., Inc. v. Merck & Co., 888 F.3d 1231 (Fed. Cir. 2018)).
• Patent misuse. Improperly extending the patent's reach, such as conditioning a license on buying unpatented goods, can bar enforcement until the misuse ends. Separately, a license term requiring royalties for use after the patent expires is unenforceable as to those royalties (Kimble v. Marvel Entm't, LLC, 576 U.S. 446 (2015)).

Assignor estoppel.

An inventor who assigned a patent, and those in privity, generally cannot later challenge its validity. The doctrine applies only when the invalidity argument contradicts what the assignor explicitly or implicitly represented in making the assignment. It does not apply, for example, when the assignee later broadened the claims (Minerva Surgical, Inc. v. Hologic, Inc., 594 U.S. 559 (2021)). Assignor estoppel does not apply at the PTAB.

Statutory exemptions.

• Government use. When the United States, or a contractor acting with its authorization and consent, uses a patented invention, the patent owner's remedy is a suit against the United States in the Court of Federal Claims for reasonable compensation (28 U.S.C. § 1498(a)). The contractor cannot be enjoined or held liable in district court for that use.
• Drug and device research. Uses reasonably related to developing information for FDA submission are not infringement (35 U.S.C. § 271(e)(1)), including preclinical studies (Merck KGaA v. Integra Lifesciences I, Ltd., 545 U.S. 193 (2005)).
• Research use. The common-law experimental use exception is very narrow. It does not protect research that furthers the user's legitimate business, including a university's research program (Madey v. Duke Univ., 307 F.3d 1351 (Fed. Cir. 2002)).

Intervening rights.

When a patent is reissued with broader claims, reexamined, or amended in an inter partes review, a party that made, bought, or prepared to use a product before the change may have the right to continue (35 U.S.C. §§ 252, 307(b), 318(c)). Absolute intervening rights cover specific items already made; equitable intervening rights are left to the court's discretion.

Texas notes.

• Customers and suppliers. When a patent owner sues both a manufacturer and its customers, courts often stay the customer suits so the manufacturer's case can go first (In re Nintendo of Am., Inc., 756 F.3d 1363 (Fed. Cir. 2014)). A customer may also have a claim against its seller under the Texas UCC warranty against infringement (Tex. Bus. & Com. Code § 2.312(c)).
• Bad-faith demand letters. Texas prohibits bad-faith patent infringement demand letters, including letters sent to end users (Tex. Bus. & Com. Code §§ 17.951-17.955). Only the Attorney General may enforce the statute; it does not create a private right to sue. See Receiving an IP Demand Letter.

Sources.

• 35 U.S.C. §§ 252, 262, 271(e)(1), 273, 286, 287, 307(b), 318(c).
• 28 U.S.C. § 1498(a).
• Tex. Bus. & Com. Code §§ 2.312(c), 17.951-17.955.
• Quanta Computer, Inc. v. LG Elecs., Inc., 553 U.S. 617 (2008).
• Impression Prods., Inc. v. Lexmark Int'l, Inc., 581 U.S. 360 (2017).
• SCA Hygiene Prods. Aktiebolag v. First Quality Baby Prods., LLC, 580 U.S. 328 (2017).
• A.C. Aukerman Co. v. R.L. Chaides Constr. Co., 960 F.2d 1020 (Fed. Cir. 1992) (en banc).
• Hyatt v. Hirshfeld, 998 F.3d 1347 (Fed. Cir. 2021).
• Hyatt v. Stewart, 148 F.4th 1376 (Fed. Cir. 2025), cert. denied sub nom. Hyatt v. Squires, No. 25-1049 (U.S. June 29, 2026).
• Google LLC v. Sonos, Inc., No. 2024-1097 (Fed. Cir. Aug. 28, 2025) (nonprecedential).
• Gilead Scis., Inc. v. Merck & Co., 888 F.3d 1231 (Fed. Cir. 2018).
• Kimble v. Marvel Entm't, LLC, 576 U.S. 446 (2015).
• Minerva Surgical, Inc. v. Hologic, Inc., 594 U.S. 559 (2021).
• Merck KGaA v. Integra Lifesciences I, Ltd., 545 U.S. 193 (2005).
• Madey v. Duke Univ., 307 F.3d 1351 (Fed. Cir. 2002).
• In re Nintendo of Am., Inc., 756 F.3d 1363 (Fed. Cir. 2014).

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