“[W]hether 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.”
“In Westinghouse Elec. & Mfg. Co. v. Formica Insulation Co., 266 U. S. 342, 349 (1924), this Court approved the ‘well settled’ patent-law doctrine of ‘assignor estoppel.’ That doctrine, rooted in an idea of fair dealing, limits an inventor’s ability to assign a patent to another for value and later contend in litigation that the patent is invalid. The question presented here is whether to discard this century-old form of estoppel. Continuing to see value in the doctrine, we decline to do so. But in upholding assignor estoppel, we clarify that it reaches only so far as the equitable principle long understood to lie at its core. The doctrine applies when, but only when, the assignor’s claim of invalidity contradicts explicit or implicit representations he made in assigning the patent.”
