摘要:Inter partes review (IPR) is one of several mechanisms for vetting patents after they have been granted. While the purpose of IPRs is to provide a cheaper, more expert alternative to litigation for screening out bad patents, the devil is in the design details. For example, the inclusion of key procedural features in IPRs, such as fixed time frames and expanded discovery contributed to making it far more popular than its predecessors. As the United States weighs additional changes to the administration of IPRs, including expanding the basis for amendment and unifying standards of review, it is worth considering the experiences of the European Patent Office (EPO) and Germany with their parallel opposition and nullification (revocation) procedures. This Article compares and contrasts U.S. IPR, EPO opposition, and German revocation actions and explores what they suggest, collectively, about the optimal design of post-grant review systems. Despite heightened concerns in the United States about IPR invalidation rates, outcomes are comparable across the three venues: 81% of reviewed claims and 26% of claims challenged in U.S. IPR proceedings are cancelled; 68% of patents reviewed and 63% of patents challenged in EPO opposition proceedings are amended or canceled; and 73% of the patents reviewed and 28% of the patents challenged in German revocation proceedings are partially or fully invalidated. But seemingly slight differences have contributed to the distinct roles each proceeding plays in its domestic patent system. The relative slowness of German infringement actions has translated into a parallel district action stay rate of 10–15%, as compared to an 80% stay rate among U.S. district court litigations that proceed in parallel to the IPR. Patentees also have fewer rights in U.S. IPRs than they do in EPO opposition proceedings, but have substantial rights to amend and can expect consolidated challenges. Focusing on how small differences have had big impacts on procedural and substantive outcomes , this Article discusses the implications of features such as the ability to amend and pre-institution decisions on the design and efficacy of post-grant patent reviews.