This Practice Note describes various US Patent and Trademark Office procedures where the USPTO may reassess the patentability of a previously issued patent. Some of the procedures, such as post-grant review, inter partes review, the transitional program for covered business method patents (sunset as of September 16, 2020), and supplemental examination were created by the Leahy-Smith America Invents Act (AIA). Other procedures, such as ex parte reexamination and reissue proceedings, have been available for many years. Counsel for both the patent owner and the patent challenger should evaluate the benefits and risks of these procedures as part of its litigation or freedom-to-operate strategy. This Note identifies key aspects of these USPTO proceedings that counsel should consider as part of the analysis.