In its recent decision in Festo Corp. v. Shoketsu Kinzoku Kogyo Kabushiki Co., Ltd (“Festo II”), the United States Supreme Court established a new balance between two significant and competing doctrines in patent law, the doctrine of equivalents and prosecution history estoppel.The long-standing balance between these two doctrines was unsettled by the decision below, in which the United States Court of Appeals for the Federal Circuit (“Federal Circuit”) rejected the “flexible bar” of its prior precedent2 as unworkable and adopted a “complete bar.” In Festo II, the Supreme Court announces a guiding principle—the imperfection of language—to govern the balancing of the competing doctrines while satisfying the incentive and notice functions of the patent system. Applying that principle, the Court rejects the complete bar established by the court below and instead adopts a foreseeability bar to govern the application of prosecution history estoppel. The foreseeability bar does
not bar all equivalents to elements narrowed by patentabilitybased amendments as did the complete bar, but does apply a sufficiently heightened standard compared to the flexible bar to be more “complete” than “flexible.” Although the new standard follows a substantially different path to determine the application and scope of estoppel than the complete bar, the foreseeability bar applies prosecution history estoppel so stringently that a limited number of cases likely will meet its strict standards for the availability of equivalents to amended claim elements.