The conventional view is that the Patent Office examines patent applications before issuance to assure compliance with the statutory criteria of patentability. Ex post invalidation in district court litigation or Patent Office cancellation proceedings then reviews the Patent Office’s work to correct errors that result from the Patent Office’s shortcomings, bias, or “rational ignorance” that limits resources spent on examination because of the irrelevance of most patents. Scholars, the Federal Circuit, and the Supreme Court have all endorsed this conventional view. However, it is wrong—or at least overly simplistic. The American patent system is only partially a system of ex ante patent examination. In other respects, it functions as a registration system where significant aspects of patentability determinations are left entirely to ex post patent invalidation in litigation and administrative proceedings. Even if the Patent Office was allocated greater resources and its examiners performed their assigned tasks perfectly, full evaluation of patentability would be impossible due to structural features of examination that exclude certain categories of prior art, prevent evaluation of the full extent of the patent owner’s exclusive rights, and allow only a snapshot evaluation of a patentability question that changes over time. Given that parts of the patentability evaluation are structurally impossible in examination, the role of ex post invalidation is more nuanced than traditionally described. In some instances, it performs a review function to correct errors in the Patent Office’s examination. In other instances, however, it serves an examination function to provide a first-instance evaluation of the aspects of patentability which are structurally unsuited for ex ante examination. Yet, the design of the patent system does not reflect the structural limits of examination or varied roles of ex post invalidation. A proper understanding of patent examination and invalidation sheds light on current debates over the presumption of validity, administrative patent cancellation, and the role of ex ante examination.