Since the America Invents Act ushered in a series of sweeping changes to the U.S. patent system in 2011, IP stakeholders have been grappling with Inter Partes Review proceedings (IPRs) before the Patent Trial and Appeal Board (PTAB). More than 8,000 IPR petitions have been filed since 2012. IPRs differ from court proceedings in many ways, including more restricted discovery, tighter deadlines, and particular rules and procedures for the submission of arguments and evidence. Despite these differences, the substantive law underlying the patent challenges is the same for IPRs and court proceedings. In an IPR, a Petitioner is allowed to challenge a patent’s validity under the doctrines of anticipation or obviousness. These same challenges are available in court, and the substantive law for anticipation and obviousness is theoretically supposed to be the same for IPRs or for court proceedings. Given that the substantive doctrines of anticipation and obviousness have existed and have been refined for more than a century, and that these doctrines are the same for PTAB and federal court proceedings, practitioners have tended to focus more on the doctrines, rules and procedures specific to IPRs when publishing on IPR practice. An in-depth study of obviousness in IPRs, however, reveals significant differences in the way the PTAB analyzes obviousness as compared to the courts and issues where the PTAB’s approach has changed over time. Over 90% of final written decisions in IPRs in the last three years have included obviousness grounds (as compared to less than 30% for anticipation).