The common-law doctrine of equivalents extends a patent’s protection to cover certain similar devices that do not literally infringe on the patent’s claims.  It is an equitable doctrine meant to protect patentees against devices whose elements perform substantially the same functions, in substantially the same ways, with substantially the same results as the patentee’s claimed elements (the “function-way-result” test).  Absent this doctrine, competitors could easily subvert the monopoly protection granted by a patent simply by substituting an unclaimed equivalent for a claimed element, thereby skirting liability for literal infringement. Such a loophole would severely threaten the patent system by “convert[ing] the protection of the patent grant into a hollow and useless thing.”  Courts have justified the doctrine as necessary to insulate a patentee from the limits inherent in using language to describe inventions. 
Broad application of the doctrine of equivalents can, however, create uncertainty as to how substantial an improvement a competitor must make to avoid non-literal infringement.  Normally, a patent’s claims set out the “metes and bounds” of the invention, just as a deed does for real property.  The claims serve a dual purpose: they both define and provide public notice of what the patent protects (and, by implication, what it does not). Both functions stimulate innovation in different ways. The definitional, or protective, function defines what aspects of the invention patent law will protect and, therefore, is the principal incentive for inventors to seek patents. The public-notice function encourages innovation by allowing competitors to make incremental improvements without fear of infringing on the claims of existing inventions. Typically, emphasizing the protective function promotes the investment necessary for breakthrough inventions, whereas emphasizing the public-notice function promotes improvements to existing technologies.  Uncertainty as to whether a patent’s claims cover an improvement can chill innovation by exposing the would-be improver to potential liability for infringement under a claim of equivalency. The doctrine of equivalents emphasizes the protective function of patents over the public-notice function because its added protection means that competitors cannot rely solely on the literal claim language to avoid creating infringing variants.
The courts created another rule, prosecution history estoppel, to cabin the expansion of protection that the doctrine of equivalents provides. Prosecution history estoppel prevents a patentee from claiming the equivalents that it surrendered during the prosecution of its patent.  Estoppel arises in such situations because a patentee disclaimed an equivalent to obtain its patent; it would be improper to permit the patentee to re-establish rights to the same equivalent through the doctrine of equivalents. Prosecution history estoppel also helps reinforce the public-notice function by permitting competitors to rely on the public patent prosecution record to help determine what the patentee has disclaimed. 
Both the U.S. Court of Appeals for the Federal Circuit and the U.S. Supreme Court have grappled with articulating the factual situations in which a patent’s specification and prosecution history precludes the added protection the doctrine of equivalents affords. The judges of the Federal Circuit hold a broad range of opinions on this issue, and the Supreme Court has had to issue repeated guidance to them.
Several opinions in these cases suggest that a patentee should not be able to invoke the doctrine of equivalents to obtain protection from equivalents that one skilled in the art would have reasonably foreseen before the patent issued. Federal Circuit Judge Rader, in particular, is a strong advocate of such a “foreseeability bar.”  The United States and the Institute of Electrical and Electronics Engineers (IEEE) also advanced the idea in their amici briefs to the Supreme Court in Festo Corp. v. Shoketsu Kinzoku Kogyu Kabushiki Co., the Court’s latest case involving the issue.  The Court’s opinion in that case also appears to limit the doctrine’s reach in cases involving amended claims to cover only those complained-of equivalents that were not foreseeable when the patentee amended its claims.
This paper examines a series of cases related to the applicability of foreseeability to the doctrine of equivalents.  It then attempts to synthesize the current state of the law in the area, and analyzes arguments for and against adopting a foreseeability rule.
Jeremy T. Marr*