This Chart sets out key differences among US Patent and Trademark Office (USPTO) proceedings where the USPTO may reassess the patentability of a previously issued patent. Inter partes review and post-grant review, which collectively replace inter partes reexamination, and the transitional program for covered business method patents, which sunset as of September 16, 2020, were created by the Leahy-Smith America Invents Act (AIA) and are conducted by the Patent Trial and Appeal Board. In addition to these AIA proceedings, the USPTO can also reassess the patentability of a previously issued patent through ex parte reexamination, reissue applications, and supplemental examination. Counsel should evaluate the differences among these proceedings before advising a client on the advantages and risks of its litigation or freedom-to-operate strategy.