In 2001, P obtained a patent relating to an apparatus and method for protecting wellhead equipment used in hydraulic fracturing. P sued Greene’s (D) for infringing that patent. D challenged the patent’s validity. D also petitioned the Board to institute inter partes review; two of the patent’s claims were unpatentable because they were anticipated by prior art not mentioned by P in its original patent application. The Board found that there was a reasonable likelihood that the two claims were unpatentable and, thus, instituted inter partes review. The proceedings before the District Court and the Board progressed in parallel. The District Court foreclosed D’s about the prior art. The Board issued a final written decision concluding that the claims were unpatentable. The Board acknowledged the District Court’s contrary decision but nonetheless concluded that the claims were anticipated by the prior art. P sought review in the Federal Circuit. P challenged the constitutionality of inter partes review. P argued that actions to revoke a patent must be tried in an Article III court before a jury. The Federal Circuit summarily affirmed the Board’s decision in this case. The Supreme Court granted certiorari.