Festo Corp. v Shoketsu Kinzoku Kogyo Kabushiki Co., 535 U.S. 722 (2002), was a United States Supreme Court decision in the area of patent law that examined the relationship between the doctrine of equivalents (which holds that a patent can be infringed by something that is not literally falling within the scope of the claims because a somewhat insubstantial feature or element has been substituted) and the doctrine of prosecution history estoppel (which holds that a party who makes a change to a patent application to accommodate the requirements of patent law cannot claim infringement by equivalents of an element that was narrowed by that change).

The Supreme Court granted certiorari, vacated, and remanded in light of an intervening decision in Warner-Jenkinson v. Hilton Davis Chemical Co.. After a decision by the original panel on remand, 172 F. 3d 1361 (1999), the Federal Circuit ordered rehearing en banc, 187 F. 3d 1381 (1999).

The Supreme Court vacated the ruling made by the Federal Circuit, holding that such amendments do not create an absolute bar, but instead must be examined in light of the reason for the change.

"The equivalent may have been unforeseeable at the time of the application; the rationale underlying the amendment may bear no more than a tangential relation to the equivalent in question; or there may be some other reason suggesting that the patentee could not reasonably be expected to have described the insubstantial substitute in question.

In those cases the patentee can overcome the presumption that prosecution history estoppel bars a finding of equivalence."