General remarks 

The EPO may be a "designated Office" or an "elected Office" for an international application filed under the Patent Cooperation Treaty (PCT) designating "EP" (Euro-PCT application). Such application is thereby deemed to be a European application (Euro-PCT application) for the purposes of the EPC. However, in the case of Euro-PCT applications, the provisions of the PCT apply in addition to those of the EPC, and where there is conflict between them, e.g. in the case of certain time limits, the provisions of the PCT prevail. According to Art. 153(1)(a), the EPO is a designated Office for contracting states to the EPC in respect of which the PCT has entered into force, which are designated in the international application and for which the applicant wishes to obtain a European patent. If the applicant has elected a designated state, the EPO is an elected Office (Art. 153(1)(b), for details see E‑IX, 2.1.1).

In addition to being a designated Office, the EPO may act as a receiving Office under the PCT within the terms set out in Art. 151. Furthermore, it may also act as an International Searching Authority (ISA), as an International Preliminary Examining Authority (IPEA) under the terms of Art. 152 and/or as a Supplementary International Searching Authority (SISA) under the PCT. There are thus the following possibilities for a European application filed under the provisions of the PCT:

the filing of the application and the international search take place at an office or offices other than the EPO (e.g. the Japan Patent Office). The EPO is a designated Office; 
the application is filed at another office (e.g. the United Kingdom Patent Office) but the EPO performs the international search. The EPO acts as International Searching Authority and is a designated Office; 
the application is filed at the EPO, which also performs the international search. The EPO acts as receiving Office, International Searching Authority and designated Office; 
in addition to the cases mentioned under (i) - (iii), the applicant files a demand for international preliminary examination. The EPO is the "elected Office"; 
the EPO is the International Preliminary Examining Authority. It may carry out this function without being the receiving Office, a designated Office or an elected Office. The EPO can, however, only act as an IPEA if the international search was carried out by the EPO, the Austrian, Spanish, Swedish, Finnish or Turkish Patent Office, the Nordic Patent Institute or the Visegrad Patent Institute. 
if the international search has been carried out by an office other than the EPO, the applicant may still request the EPO to perform a supplementary international search (SIS) in its capacity as SISA. 

In case (i), the European application will be accompanied by an international search report drawn up by another office. In cases (ii) and (iii), the international search report and the "written opinion of the International Searching Authority" (WO-ISA) (Rule 43bis PCT) will be prepared by the search division of the EPO. In case (iv), the international search report and the international preliminary examination report may be drawn up by the EPO or by another International Searching Authority and International Preliminary Examining Authority.

For further details, in particular as to deadlines and procedural steps before the EPO as RO, ISA, IPEA or SISA, see the latest version of the Guide for applicants: "'Euro-PCT Guide': PCT procedure at the EPO".

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