Assignor estoppel

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Assignor estoppel is the patent law doctrine that an assignor of a patent is estopped, or barred, from later contesting the validity of the patent. The logic behind this judge-made doctrine is that by assigning the patent, the patent-holder accepted its validity, and cannot contradict himself on that point later.

The America Invents Act abrogated this doctrine for inter partes proceedings,[1] however, which further undermines the validity of patents like much of what the AIA has done.

On January 8, 2021, the U.S. Supreme Court granted cert. in No. 20–440 to consider this question:


In the Patent Act, Congress established that invalidity is a “defense[] in any action involving the validity or infringement of a patent.” 35 U.S.C. § 282(b) (emphasis added).

There is no textual exception to this command. The Federal Circuit nonetheless applies a judge-made “equitable” exception to the statute’s unqualified language known as “assignor estoppel.” Assignor estoppel prevents an inventor who has assigned a patent from later contesting the patent’s validity.

The question is whether a defendant in a patent infringement action who assigned the patent, or is in privity with an assignor of the patent, may have a defense of invalidity heard on the merits.