The past few months have shed some light on the Patent Trial and Appeal Board’s approach to estoppel in post-grant proceedings, which applies to final written decisions on a claim-by-claim basis. Moreover, estoppel applies not only when a petitioner, or a real party in interest or privy of the petitioner, raised the arguments in a prior IPR or PGR proceeding, but also when he/she “reasonably could have raised” them.