The Italian Socialist Party was a social-democratic and democratic-socialist political party in Italy, whose history stretched for longer than a century, making it one of the longest-living parties of the country. Founded in Genoa in 1892, the PSI was from the beginning a big tent of Italy's political left and socialism, ranging from the revolutionary socialism of Andrea Costa to the Marxist-inspired reformist socialism of Filippo Turati and the anarchism of Anna Kuliscioff. Under Turati's leadership, the party was a frequent ally of the Italian Republican Party and the Italian Radical Party at the parliamentary level, while lately entering in dialogue with the remnants of the Historical Left and the Liberal Union during Giovanni Giolitti's governments to ensure representation for the labour movement and the working class. In the 1900s and 1910s, the PSI achieved significant electoral success, becoming Italy's first party in 1919 and during the country's Biennio Rosso in 1921, when it was victim of violent paramilitary activities from the far right, and was not able to move the country in the revolutionary direction it wanted.

The Left group, later called Historical Left by historians to distinguish it from the left-wing groups of the 20th century, was a liberal and reformist parliamentary group in Italy during the second half of the 19th century. The members of the Left were also known as Democrats or Ministerials. The Left was the dominant political group in the Kingdom of Italy from the 1870s until its dissolution in the early 1910s.