From 2008 to 2014, the Council on Library and Information Resources (CLIR) administered the Cataloging Hidden Special Collections and Archives program, which granted funds to catalog "hidden" collections of high scholarly value. Generously supported by The Andrew W. Mellon Foundation, the program awarded more than $27.4 million to academic, cultural heritage, and other collecting institutions. To assess the program's impact, CLIR conducted a comprehensive analysis of final reports from all 128 projects funded through the program. This report describes the methods and findings of the analysis, including cataloging outputs, as well as impact on hiring, policies and procedures, communication tools, and research and outreach. The findings of this report are divided into four sections: an overview of project demographics, a summary of project outputs, a discussion of other outcomes, and an examination of the lasting impact of the program.