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). If an applicant enters the European phase without having requested international preliminary examination under PCT Chapter II, the EPO will act as a "designated Office". If before entering the European phase the application was processed under PCT Chapter II, the EPO will act in the European phase as an ''elected Office''. Pursuant to Art. 153(2), an international application for which the EPO is a designated or elected Office is deemed to be a European patent application.[Art. 153(1)(a); Art. 153(1)(b); Art. 153(2); Art. 150(2); ]

In addition to being a designated and, where appropriate, elected Office, the EPO may act as a receiving Office under the PCT within the terms set out in Art. 151. 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 an International Searching Authority specified for Supplementary International Search (SISA) under the PCT (see also the EPO-WIPO Agreement, OJ EPO 2017, A115, OJ EPO 2018, A24, and OJ EPO 2018, A35). There are thus the following possibilities for a European application filed under the provisions of the PCT:[Art. 151; Art. 152; Rule 157; Rule 158; ]

In case (i), there will be 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 have been prepared by the search division of the EPO. For further details on the procedure before the EPO as RO, ISA, IPEA or SISA, see the Guidelines for search and examination at the EPO as PCT authority (GL/PCT‑EPO) and the Guide for applicants: "'Euro-PCT Guide': PCT procedure at the EPO".