The International Instituut Voor Sociale Geschiedenis, or International Institute of Social History, located in Amsterdam, is well known as one of the world's leading centers for research on the history of labor movements and leftist political parties. It is also a major depository of archival material. However, scholars have largely neglected the Institute's extraordinary collection of Latin American materials. The purpose of this article is to describe the two largest bodies of Latin American holdings, those pertaining to Argentina and Brazil. For reasons of space, we have kept annotations to a minimum and have included only materials published before 1940, since they constitute by far the most significant portion of the Institute's Latin American collection.