When Congress implemented inter partes review (IPR) and other patent post-grant proceedings through the passage of the America Invents Act (AIA) in 2011, it provided that petitioners would be estopped in later proceedings from raising grounds for invalidity that they “raised or reasonably could have raised during that inter partes review.” 35 U.S.C. § 315(e)(2). However, substantial uncertainty in interpretation of this provision causes an enormous impact on an accused patent infringer’s decision of whether and on what grounds to petition for review. One reading of the statutory estoppel provision suggests that “during that inter partes review” refers to the time period after institution of the IPR and before a final decision from the Patent Trial and Appeal Board (PTAB). However, although the Federal Circuit has generally favored a textualist reading of this statute, it has yet to address how the estoppel standard would apply to grounds that the petitioner chose not to include in the petition for IPR. Lower courts have taken widely divergent approaches in this and various other factual scenarios where estoppel might apply, with some courts assessing legislative history to determine that Congress intended estoppel to apply broadly to any grounds that could have been raised in the petition. These courts often engage in complex assessments of what prior art the petitioner should have known about and added to the petition, which adds unnecessary complexity and cost to their determinations. This Article will explain that a narrow, textualist reading of the IPR estoppel provision is consistent with Supreme Court precedent on statutory interpretation and is justified in light of the strict limits on IPR petitions adopted by the PTAB—limits that Congress did not foresee and which effectively restrict the grounds for invalidity that a petitioner may raise during the IPR. When the Federal Circuit or the Supreme Court next has occasion to consider this issue, it should hold that IPR estoppel applies narrowly only to grounds actually raised and instituted.