Once a Patent Cooperation Treaty (PCT) application is filed, the applicant needs to select an International Search Authority (ISA) to perform a search of prior art and to issue a Written Opinion giving its opinion on the patentability of the claim as filed.

Patent offices including the United States Patent and Trademark Office (USPTO), European Patent Office (EP), Korean Intellectual Property Office (KIPO), and at least 12 others, serve as ISAs. Which ISAs are available to an applicant depends on where the PCT was filed, and the applicant will want to consider quality, cost, and speed when selecting which ISA to use.