Web-scale discovery services such as Summon (Serial Solutions), WorldCat Local (OCLC), EDS (EBSCO), and Primo (Ex Libris) are often touted as a single search solution to connect users to library-owned and -licensed content, improving discoverability and retrieval of resources. Assessing how well these systems achieve this goal can be challenging, however. This article describes the use of interlibrary loan borrowing request data as a means to evaluate whether the Summon discovery system was achieving its goal of improving retrieval. Four years following implementation of the discovery system, there was a 22 percent decrease in interlibrary loan borrowing requests. Requests by undergraduates dropped at a higher rate than faculty, staff, and graduate students. Undergraduate requests for articles dropped more than requests for loans. Two years' postimplementation of the discovery system, the number of interlibrary loan borrowing requests by undergraduates for locally owned or licensed materials dropped by 57 percent, freeing up interlibrary loan staff to expand services into new areas. This study indicates that implementation of a discovery system can lead to a reduction in interlibrary loan borrowing.