By controlling and changing how antimicrobial agents are selected and administered, antimicrobial stewardship programs (ASPs) aim to prevent or slow the emergence of antimicrobial resistance; optimize the selection, dosing, and duration of antimicrobial therapy; reduce the incidence of drug-related adverse events; and lower rates of morbidity and mortality, length of hospitalization, and costs. There is an abundant and growing body of evidence demonstrating that ASPs change the quantity and quality of antimicrobial prescriptions; however, measuring whether, when, and how ASPs improve patient outcomes and change patterns of antimicrobial resistance--which is the ultimate goals of ASPs--has been difficult, but the totality of evidence indicates that ASPs are capable of achieving these goals. In this article, we review the existing data on ASPs and their effects on patient care and antimicrobial resistance, as well as strategies for establishing ASPs in different types of hospitals.