A Patent Challenger (sometimes referred to as an accused infringer) is a litigant in a Patent Case who is accused of infringing a U.S. patent or who is challenging the infringement, validity or enforceability of a U.S. patent. In district court cases a Patent Challenger is usually a defendant, but may be a plaintiff in declaratory judgment cases (i.e., cases in which the complaint seeks a declaration of patent noninfringement, invalidity or unenforceability). In the ITC a Patent Challenger is usually a respondent. In the PTAB a Patent Challenger is the petitioner.