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). Background of the case Festo Corporation (petitioner) possessed patents for an industrial device. After Festo began marketing its device, Shoketsu Kinzoku Kogyo Kabushiki Co. (SMC, respondents) entered the market with a device that used one two-way sealing ring and a nonmagnetizable sleeve. Festo Corporation already owned two similar patents (although their initial patent application was rejected) for this industrial device. Festo filed suit, claiming that SMC's device was sufficiently similar that it infringed Festo's patents under the doctrine of equivalents. Festo's claim had been amended during prosecution for, at the very least, compliance with 35 U.S.C. §112 (claim structure), and thus Shoketsu claimed that prosecution history estoppel should bar Festo from asserting equivalents. The United States District Court for the District of Massachusetts held that Festo's amendments were not made to avoid prior art, and therefore the amendments were not the kind that give rise to estoppel. A panel of the Federal Circuit affirmed. 72 F. 3d 857 (1995). 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 court sitting en banc held that claim amendments made for compliance with the Patent Act presented a complete bar to claiming equivalents. Issue Whether any amendment to a patent application that narrowed a patent claim to comply with the Patent Act creates an absolute bar to equivalents for the particular claim limitation that was narrowed by the amendment. Court's decision 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