Inter partes review is a trial proceeding conducted at the Patent Trial & Appeal Board (Board) at the United States Patent & Trademark Office (USPTO) to re-investigate the patentability of one or more claims in an “already-issued” patent only on a ground that could be raised under 35 U.S.C. §§ 102 or 103, and only on a basis of prior art consisting of patents or printed publications.  Typically, a Defendant who is sued for patent infringement in a US District Court has an option to file a petition for inter partes review (IPR) before the PTAB, and then moves stay the district court case until the PTAB renders a decision on the validity of the claim in the patent in suit.  Challenging the patent asserted against it in the district court litigation is a great play for the defendant therein, as the IPR is amazingly cheap when compared to a patent litigation due to the the limited discovery available in the IPR, because the IPR levels the playing field between Plaintiff (Patent Owner/Respondent in the IPR) and Defendant (Petitioner in the IPR), i.e., the presumption of validity and the enhanced burden of proof required to invalidate a patent in the Article III Court does not apply to the IPR before the PTAB.  But even if you are Plaintiff (Patent Owner), there is value in getting through validity issues in a financially expedient way, up front (i.e., in the IPR).  If Plaintiff’s (Patent Owner/Respondent in the IPR) patent makes it through the IPR, Defendant/Petitioner can’t challenge validity in the district court case once the stay is lifted, post IPR.  But whether an IPR should be brought (from a Defendant’s perspective) really depends on whether their is some prior art out there that could affect the validity of the patent in play.  Call your patent attorney!!!!