Located in Amsterdam, its one million volumes and 2,300 archival collections include the papers of major figures and institutions in radical leftist thought.

[4] To examine how labour relations develop over time, IISG collected archives from all over the world.

During the first years Posthumus succeeded in obtaining many papers from anarchists (Bakunin manuscripts), other socialist and social democratic and Marxist movements from Germany and Russia.

Most of the papers were rediscovered in Hannover in 1946, and some other parts were later found in archives in Moscow in 1991, and returned to Amsterdam.

[5] In 1989 the International Institute of Social History moved to new premises: an old warehouse at the Cruquiusweg in the eastern part of Amsterdam.