Inter Partes Review (IPR): This is a process where a third party can challenge the validity of a patent based on prior art. It is one of the most common procedures at the PTAB. Post Grant Review (PGR): This allows challenges to the validity of a patent on broader grounds than IPR, but it must be filed within nine months of the patent grant. Covered Business Method Review (CBM): This is specific to business method patents and allows for challenges based on prior art or certain patentable subject matter issues.