A federal trademark registration is a powerful asset, but it is a rebuttable presumption, not a fortress: a court can strike it and order the federal register corrected when the mark should never have registered or should no longer stand. This guide explains cancellation with a focus on the district court's power under Section 37 of the Lanham Act, 15 U.S.C. § 1119, and compares that route with a Trademark Trial and Appeal Board petition and the newer ex parte expungement and reexamination proceedings created by the Trademark Modernization Act. It maps the grounds against a registration's age and incontestability, then works through the doctrines that decide real cases: entitlement after Lexmark, the demanding fraud standard after In re Bose, abandonment and nonuse against a famously low use-in-commerce threshold, genericness, functionality, intent-to-use defects, and the surname bar. It addresses which grounds can be won on summary judgment, the preclusive reach of Board decisions after B&B Hardware, and the court's power to rectify the register in whole or in part. A fully worked hypothetical and separate checklists of takeaways for challengers and registrants close the analysis.