International organizations figure prominently in the collections at the IISH. The rise of early workers organizations was closely linked to labour migration both within Europe and among countries in Europe and other nation states. The workers movement tried to inspire international solidarity by furthering socialism and trade unionism. Sources relating to the Internationals - from the First to the Fourth and beyond - are core collections at the IISH. Several classics among the archives, for example Karl Marx, Friedrich Engels, Karl Kautsky and Moses Hess, reflect the historical impact of socialist theory far beyond national borders. The IISH holds the records of various international trade union federations, student groups, and grassroots peace organizations. Amnesty International, Greenpeace International, and other new social movements have entrusted their collections to the Institute.
The IISH reference library aims to provide a comprehensive variety of perspectives on the history of international labour organizations and global social history since the early modern period. Special library collections included in the IISH library have broadened its scope considerably. These collections include the autonomous and anti-globalist movements, alternative economy, ecology, the women's movement, and the gay rights movement worldwide.
All collections and collection items are retrievable via the IISH catalogue. The Institute is digitizing its leading archives and collections. The archives of Trotsky's International Left Opposition, the Jewish Bund and many others are now fully available to view via the Institute’s catalogue.