Festo (Plaintiff) owned two patents directed to magnetic conveyor systems, each of which were amended during the patent application process. Festo sued Shoketsu (Defendant) for infringement. Shoketsu argued as a rule that amendments made during patent prosecution which narrowed the scope of the claims barred the application of the doctrine of equivalents to those claims Because Festo had narrowed certain elements of its claims through amendment, Shoketsu argued, the doctrine of equivalents could not be applied those claim elements. The district court found for Shoketsu, which was affirmed by the Court of Appeals for the Federal Circuit, and then overturned by the United States Supreme Court. On remand, the Federal Circuit en banc ruled against Festo, and the case was again appealed to the United States Supreme Court.