Over the past century, few patent issues have been considered so often by the Supreme Court of the United States as the doctrine of equivalents (“DOE”). This judge-made rule deals with a question that lies at the heart of patent policy—what is the best way to define property rights in an invention? The doctrine gives patentees an opportunity to ensnare an accused device that does not literally infringe a patent claim if the accused device is substantially similar to each claim limitation. Patentees enjoy this advantage, but it comes at a cost to the public, who must face the uncertainty of whether claims actually mean what they say. This tension chafed the Justices and split the Court almost down the middle in two early cases. From those controversial beginnings to the present day, judges, practitioners, and academics continue to debate the doctrine’s proper scope and continued vitality.