An International Application  filed under The Patent Cooperation Treaty (PCT) (colloquially referred to as a “PCT application”), is filed in a receiving office (RO) and the claimed invention undergoes a prior art search by an International Searching Authority (ISA) . The ISA is chosen by the Applicant,  and may be any one of the following:

• The United States Patent and Trademark Office (USPTO)
• the European Patent Office (EPO),
• the Korean Intellectual Property Office (KIPO),
• the Australian Patent Office (IP Australia) (IPAU),
• the Federal Service for Intellectual Property (Rospatent) (Russian Federation),
• the Israel Patent Office (ILPO),
• the Japan Patent Office (JPO), and
• the Intellectual Property Office of Singapore (IPOS).

 
The selected ISA then conducts a search for the relevant prior art in view of the claims in the PCT application.  The ISA also prepares an
international search report (ISR); and (3) a Written Opinion (WO).

The Written Opinion of the ISA is considered the first written opinion of the International Preliminary Examining Authority (IPEA). The Written Opinion provides the Examiner’s position as to whether each claim is "novel," involves an "inventive step," and is "industrially applicable".  As noted below, the choice of ISA can have a strategic importance for the examination of the IA.
 
A response to the Written Opinion is not required. If not responded to, an international preliminary report on patentability issues, which contains the same rejections as the Written Opinion.  The PCT application then enters national phase in the countries selected by the Applicant having this as the only opinion on the patentability of the claims.
 
If an Applicant files a response (and optionally amendment to the claims) along with a Demand and payment of the required fee, then the Examiner reviews the response and any amendment filed and prepares a further report regarding the patentability of the claims.
 
Choosing to respond to the Written Opinion may be of strategic advantage. If Applicant’s goals regarding its PCT application, and the claim scope of interest, are known, it may be advantageous to respond and amend the claims to a desired scope and to exclude the cited prior art from the claims. Because the response is not generally due until 22 months from the priority date*, the Applicant typically has more than 3 months  after receiving the Written Opinion to develop a strategy for responding to the rejections and for amending the claims.

A response to the Written Opinion requires filing a Demand. Applicant should submit the Demand and appropriate fees directly to the ISA who prepared the Written Opinion. By filing a Demand, the Applicant obtains an additionalround of examination of the application, which allows the Applicant tofile amendments to the description, claims and drawings to correct any defects, respond to any observations, or address rejections raised in the Written Opinion with respect to any of the claims because of a lack of novelty, inventive step (non-obviousness) or industrial applicability. This initiates examination and enables the Applicant to attempt to obtain a positive international preliminary examination report.  A positive International Preliminary Report of Patentability (IPRP) can be used to accelerate prosecution of the corresponding national applications.  For example, in a US national phase application, an Applicant with a positive IPRP can accelerate prosecution without paying any additional fees using the Patent Prosecution Highway (PPH).  Similar options exist in other national offices which are members of the Global PPH and the IP5 PPH, such as Canada, Australia, Israel, Japan, Korea, European Patent Office, and China.