In this first large-scale study of the effect of discovery systems on electronic resource usage, the authors present initial findings on how these systems alter online journal usage by academic library researchers. The study examines usage of content hosted by four major academic journal publishers at 24 libraries that have implemented one of the major discovery systems, EBSCO's EDS, Ex Libris’s Primo, OCLC’s Worldcat Local, or SerialsSolutions’s Summon. A statistically rigorous comparison of COUNTER-compliant journal usage at each library from the 12 months before and after implementation will determine the degree to which usage rises or falls after discovery tool implementation and address rumors that discovery tools differ in their impact on electronic resource usage.