[11] The PES comprises national-level political parties from all the European economic area states (EEA) plus the United Kingdom.

The Socialists' 1962 congress pushed for greater democratisation and powers for Parliament, though it was only in 1969 that this possibility was examined by the member states.

[14] In 1973, Denmark, Ireland and the United Kingdom joined the European Community, bringing in new parties from these countries.

The party's first programme concentrated on job creation, democracy, gender equality, environmental and consumer protection, peace and security, regulation of immigration, discouragement of racism and fighting organised crime.

He was replaced as interim president by Sergey Stanishev, at the time chairman of the Bulgarian Socialist Party (BSP) and former prime minister of Bulgaria.

On 28–29 September 2012, the PES Congress in Brussels[17] Congress elected interim president Sergey Stanishev as full President, as well as four deputies: Jean-Christophe Cambadélis (1st Vice-President – PS), Elena Valenciano (PSOE), Jan Royall (Labour) and Katarína Neveďalová (Smer-SD).

The Congress also approved Achim Post (SPD) as the Secretary-General as well as the four Vice-Presidents: Jean-Christophe Cambadélis (PS), Carin Jämtin (Swedish Social Democratic Party), Katarína Neveďalová (Smer-SD) and Jan Royall (Labour).

Sergey Stanishev was confirmed as party President and Achim Post (SPD) as secretary general.

The Election Congress acclaimed European Commission First Vice-President Frans Timmermans and adopted its manifesto: A New Social Contract for Europe.

[19] On 16 December 2021, the PES held its Council in Brussels, adopting the resolution: Fairness, Sustainability, Respect: a progressive vision for the future of Europe.

[28] In December 2009, the PES decided to put forward a candidate for Commission President at all subsequent elections.

Through its associate and observer parties the PES has eight heads of state or government in non-EU countries: Party-alignment at the European Council is often loose, but has been the basis of some intergovernmental cooperation.

At present five countries are led by a PES-affiliated leader, who represents that state at the European Council: Germany (Olaf Scholz), Spain (Pedro Sánchez), Malta (Robert Abela), and Denmark (Mette Frederiksen).