The Balch Institute was a library and museum dedicated to immigration and ethnic history, based in Philadelphia. This article analyzes the development of its print and manuscript collections, from its first accessions in 1971 to its merger with the Historical Society of Pennsylvania in 2001. It focuses on its dual scholarly and educational mission and on the factors that shaped its collection development policies and practices over time. The article argues that the content of the collections, but also what was not included in them, was determined by the institutional and broader political, social, and scholarly context in which the Balch operated. The Balch’s contribution to the historical record illustrates the importance of historicizing the archives so that historians have deeper knowledge of the role played by the professionals who acquire and manage the sources they use.