Antimicrobial stewardship programs (ASPs) exist to optimize antibiotic use, reduce selection for antimicrobial-resistant microorganisms, and improve patient outcomes. Rapid and accurate diagnosis is essential to optimal antibiotic use. Because diagnostic testing plays a significant role in diagnosing patients, it has one of the strongest influences on clinician antibiotic prescribing behaviors. Diagnostic stewardship, consequently, has emerged to improve clinician diagnostic testing and test result interpretation. Antimicrobial stewardship and diagnostic stewardship share common goals and are synergistic when used together. Although ASP requires a relationship with clinicians and focuses on person-to-person communication, diagnostic stewardship centers on a relationship with the laboratory and hardwiring testing changes into laboratory processes and the electronic health record. Here, we discuss how diagnostic stewardship can optimize the "Four Moments of Antibiotic Decision Making" created by the Agency for Healthcare Research and Quality and work synergistically with ASPs.