A Practice Note discussing estoppel under 35 U.S.C. § 315(e) that may attach in related proceedings to unpatentability grounds a petitioner previously raised (or could have raised) in an inter partes review proceeding (IPR) before the Patent Trial and Appeal Board (PTAB). It addresses the legal framework and practical considerations for litigating a Section 315(e) estoppel defense, including raising estoppel and obtaining relevant discovery to support or refute it.