Party of European Socialists (PES) was formed in 1992 when the EU Treaty added to the Treaty establishing the European Community Article 138a (later renamed as Article 191) recognizing the political parties on European level as important factors of integration within the EU with the object to pursue international aims in respect of the principles on which the EU is based. The more specific aims are reflected in the Statutes adopted by the 8th PES Congress in December 2009.

PES succeeded the Confederation of Socialist Parties of the European Community, which had been set up in 1974. But PES considers as its beginning the year of 1957, the years of 1950 as the beginning of the Europea Socialist Movement; and the year of 1864 as the beginning of cooperation between European Socialist parties (Hix, S., Lesse, U. (2002). Shaping a Vision. A History of the Party of European Socialists. Brussels: Party of European Socialists, 9–11).

PES brings together 33 Socialist, Social Democratic and Labour Parties from 27 the European Union Member States and Norway. In addition, twelve associate and five observer parties belong to PES. Of political groups in EU institutions belong to PES Socialists & Democrats in the European Parliament and PES Group in the Committee of the Regions. The sectoral organizations in the PES are PES Women, Young European Socialists (ECOSY), and Foundation for European Progressive Studies (FEPS); the associated organizations are The Socialist International, Socialist Group in the Parliamentary Assembly of the Council of Europe, and Socialist Group in the Parliamentary Assembly of the OSCE. The observer organizations are Socialist International Women (SIW), International Union of Socialist Youth (IUSY), European Forum for Democracy and Solidarity, The Joint Committee of the Nordic Social Democratic Labour Movement (SAMAK), International Falcon Movement - Socialist Educational International, Rainbow Rose LGBT Network, Union of Socialist Local and Regional Representatives in Europe (USLRRE), European Senior Organization, and International Social Democratic Union for Education.

The PES is politically represented in the directly elected European Parliament where the PES fraction has 162 + 5 seats (Bor­chardt 2010, 51), at governmental level in the European Council and Council of Ministers, and in the Committee of the Regions. In the European Commission six of the Commissioners have roots in the PES - the Vice-President of the European Commission and High Representative of the Union for Foreign Affairs and Security Policy Catherine Ashton, the Vice-President of the European Commission and the Commissioner for Competition Joaquín Almunia, the Vice-President of the European Commission and the Commissioner for Inter-Institutional Relations and Administration Maroš Šefčovič, the Commissioner for Maritime Affairs and Fisheries Maria Damanaki, the Commissioner for Enlargement and European Neighbourhood Policy Štefan Füle, and the Commissioner for Employment, Social Affairs and Inclusion László Andor - although the Commission is an independent, collegiate body.

As for the presence of the PES parties in government at national level see http://www.pes.org/en/about-pes/pes-eu/members-government-and-heads-state who represent in the European Council and the Council of the European Union the interests of both State and EU, and here the choice of the State representative depends on the elections in the State.

The Member States' presidents with the roots in the PES are for example Toomas Hendrik Ilves, and Tarja Halonen.

As for the aims and activities of the PES see: http://www.pes.org/en

From Estonia belongs to the PES the Estonian Social Democratic Party (Eesti Sotsiaaldemokraatlik Erakond).