In 2014, the Marian Library at the University of Dayton completed a long overdue revision of its collection development policy. The new document more clearly defined the scope of the library’s collections, and was intended to guide new acquisition decisions. However, this new document had the unexpected benefit of providing a framework for deselection projects that enabled preservation and improved access to the collections. This paper will discuss and analyze two of these projects, and demonstrate how the revised collection development policy laid the foundation for successful deselection outcomes. In the first case study, legacy collections of genre-based ephemera were heavily weeded to remove photocopies, internet printouts, duplicates, and other out-of-scope materials. Both the challenges and benefits of weeding legacy reference files will be discussed. The second case study will examine a comprehensive review of the library’s inactive periodical holdings, consisting of over five hundred titles that were largely uncatalogued. Removing titles outside of the library’s collection scope transformed the collection into a manageable project for the cataloging staff to tackle. This formerly hidden collection, including rare periodicals not found elsewhere in the United States, is now in the process of being cataloged. Both projects transformed local practices and improved utilization of the library’s limited resources in staffing, time, space, and funding. Faced with legacy practices that compromised physical and intellectual control of materials, librarians leveraged a well-defined collection development policy to undertake two successful deselection projects. The policy was used to justify and guide deselection, ultimately improving both preservation and access.