Amendment that narrows claim in order to comply with any provision act, including 35 U.S.C. § 112, may invoke Prosecution History Estoppel; this rule includes “voluntary” amendments. Further, amendment that narrows claim is presumed to have been made for “substantial reason related to patentability, if record does not reveal reason for amendment; patentee may rebut presumption by showing that amendment was not one relating to patentability, but this rebuttal is restricted to evidence in prosecution history record. Still further, finding that amendment which narrowed claim was made for substantial reason related to patentability imposes presumption that patentee had surrendered all territory between original claim limitation and amended limitation; if patentee fails to rebut this presumption, then Prosecution History Estoppel bars patentee’s reliance on Doctrine of Equivalence for accused element, but if patentee successfully rebuts presumption, then question of whether accused element is in fact equivalent is reached on merits. The three criteria for rebutting the Festo presumption are: (1) patentee to show that an alleged equivalent would have been “unforeseeable at the time of the amendment and thus beyond the fair interpretation of what was surrendered”; (2) patentee to demonstrate that “the rationale underlying the narrowing amendment bore no more than a tangential relation to the equivalent in question”; or (3) patentee to establish “some other reason suggesting that the patentee could not reasonably be expected to have described the insubstantial substitute in question”. (Festo Corp. v. Shoketsu Kinzoku Kogyo Kabushiki Co., 68 USPQ2d 1321, CA FC, 9/26/03).