This question might be rather academic. It is about patent litigation which is related to the US patent system.

If a patent owner thinks, his patent is being infringed, the patent owner may choose to sue the infringing party to stop his or her activities, as well as to receive compensation for the unauthorized use.

To initiate litigation, patent owner must file a complaint, listing the alleged infringer, the patent(s) allegedly infringed, the infringing products and request relief. This is as simple as filling out Form 18 provided by the US courts and filing it with a district court.

The defendant responds to the plaintiff's complaint by preparing an answer denying the allegations asserted against it, stating defenses to each claim asserted against it and usually asserting counterclaims.

Common defenses are:

  • the complaint does not state a claim upon which relief can be granted.
  • the products do not infringe the patents-in-suit (35 USC §282(b))
  • the patents-in-suit are invalid
  • laches
  • ...and others...

According to FRCP rule 8, there are "defenses" and "affirmative defenses".

Affirmative defenses in nature admit the claims asserted by plaintiff but provide other facts which defeat or mitigate the legal consequences.

There are many answers to complaints where all defenses are listed in a section titled "affirmative defenses", also the non-infrigement defense e.g.:

But how can asserting "non-infringement" be an affirmative defense when the cause of action is infringement? "Non-infringement" is a defense, but a "non-affirmative" defense in my opinion.

3 Answers 3


It's not formaly an affirmative defense under the law. Lawyers throw it in as a cya for some reason. Affirmative defenses generally track the burden of proof. The patent plaintiff has the burden of proof on infringement so the defendant doesn't have to present the "affirmative defense" of noninfringement, but merely has to deny it. The defendant generally has the burden of proof on invalidity so that is generally plead as an affirmative defense.


I think that's the entire point.

The plaintiff alleges that the product infringes the patent. But allegation is not proof of fact, whatever the tabloid newspapers wish you to believe. If the product does not in fact infringe the patent, then there is no tort and no damage to rectify, and the defendant must be given a chance to show that.

If non-infringement were not an affirmative defence, then any old troll could extract damages at will - simply by holding a patent and alleging infringement of it, without any evidence whatsoever. The legal system is intended to prevent such extortion, even if it isn't always very effective at it.

More commonly, a defendant may wish to show that, although they have a product which achieves the same goal as the patented invention, the product does so in a different way than the patent describes and has claims for - eg. an eccentric cam versus a crankshaft, to quote a common nineteenth-century example. This may not be obvious from the outside, especially with microelectronics and software.

  • 1
    I agree with you, @Chromatix. My question was related to the fact that some attorneys list "non-infringement" under "affirmative defenses" although its a "non-affirmative" defense in my opinion. Asserting "non-infringement" does not affirm anything.
    – Daniel K.
    Commented Nov 24, 2013 at 10:52
  • 2
    Ah, I see the distinction now. But perhaps it merely affirms that the patent exists, while denying that it covers the product? Honestly, it's not a distinction I'm familiar with, since I have read mostly about the English legal system, where a defence is simply a defence.
    – Chromatix
    Commented Nov 24, 2013 at 14:24

I just listened to the Supreme Court oral arguments in Medtronic v Boston Scientific and non-infringement as an affirmative defense came up. The attorney being questioned made a distinction between a basic denial of infringement and something that acknowledged that even if actions might might fall within the words of the claim they did not constituent infringement because it fell into a judicially created exception for experimental use or because prosecution estoppel prevented the patentee from a particular claim construction. None of the justices argued with him about that. I suppose another case could be "we did it but we have an implied license".

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