35 U.S.C. § 282 — Presumption of validity; defenses

(a) In General. A patent is presumed valid. Each claim in a patent — whether independent, dependent, or multiple dependent — is presumed valid on its own, even if it depends on a claim that turns out to be invalid. Whoever argues a patent or claim is invalid must prove it. (b) Defenses. In a lawsuit about a patent's validity or infringement, these defenses must be raised in the pleadings: (1) not infringing, no liability for infringement, or unenforceability; (2) the patent or claim is invalid under any ground listed in Part II of this title for getting a patent; (3) the patent or claim is invalid because it didn't meet (A) any requirement of section 112 — except that failing to disclose the "best mode" cannot be used to cancel or invalidate a claim — or (B) any requirement of section 251; and (4) any other fact or act this title makes a defense. (c) Notice of Actions; Actions During Extension of Patent Term. A party arguing a patent is invalid or not infringed must give written notice at least 30 days before trial, listing the country, number, date, and patentee's name of any patent, and the title, date, and pages of any publication, they plan to use as proof the invention was already known, or (except in the Court of Federal Claims) as proof of the state of the art. They must also name any person they'll rely on as the true prior inventor or as having known about, used, or offered to sell the invention first. Without this notice, the court won't allow proof of these things at trial except on terms the court sets. Separately, if a patent term extension under section 154(b) or 156 is invalid because the applicant or the Director failed to follow that section's requirements, that's a defense in an infringement suit during the extended period, and it must be pleaded. A due diligence decision under section 156(d)(2) cannot be reviewed in that kind of lawsuit.