Inter partes review is the administrative trial that reshaped American patent litigation: a fast, expert-adjudicated proceeding in which anyone other than the patent owner can ask the Patent Trial and Appeal Board to cancel issued claims on prior-art grounds, under a preponderance standard and with no presumption of validity. This UpLaw guide walks the entire arc — choosing among IPR, post-grant review, and derivation; the nine-month PGR window and the unforgiving one-year bar of 35 U.S.C. § 315(b); real-party-in-interest and privity disclosures that can sink an otherwise strong petition; drafting to the reasonable-likelihood standard inside a 14,000-word cap; the patent owner's preliminary response; institution and the all-claims, all-grounds rule of SAS Institute v. Iancu; discretionary denial under Section 314(a), the Fintiv factors, and Section 325(d); claim construction under Phillips; the trial phase, cross-examination, and the motion to amend; the final written decision; and the estoppel of Section 315(e) that follows the petitioner back into district court. It closes with Federal Circuit appeal, Article III standing to appeal, the Appointments Clause fallout from United States v. Arthrex and the Director-review regime it produced, and the strategic question that drives most of this practice: whether the district court will stay the parallel infringement case. A running hypothetical follows Kestrel Devices, an accused infringer racing the one-year clock, from service of the complaint through final written decision and appeal. The law here is entirely federal — Title 35, 37 C.F.R. Part 42, and the Federal Circuit — but the Office's discretionary-denial policy has changed repeatedly, and this guide flags where to verify current practice.