The use of antibiotics in both ambulatory and inpatient settings is heavily shaped by cultural and economic factors as well as by microbiological considerations. These nonpharmacologic factors are relevant to clinicians and policymakers because of the clinical and fiscal toll of inappropriate antibiotic prescribing, including excessive use, preventable adverse effects, and the increasing prevalence of resistant organisms. An understanding of the determinants of antibiotic consumption is critical to explain current patterns of use and to devise programs to reduce inappropriate use. Patient motivations include the desire for a tangible product of the clinical encounter coupled with incorrect perceptions of the effectiveness of antibiotics, particularly in viral infections. Physician behavior can be explained by such factors as lack of information, a desire to satisfy patient demand, and pressure from managed care organizations to speed throughput. Marketing campaigns directed at both physicians and patients further serve to increase demand, especially for newer, costlier products. Studies of antibiotic use patterns in inpatient and outpatient care consistently demonstrate considerable inappropriate prescribing, which is likely to exacerbate the emergence of resistant organisms. Several approaches have been shown to improve the rationality of antibiotic use. Computer-based algorithms or reminders can prompt physicians to improve antibiotic choices at the time of prescribing; paper-based order entry forms can achieve the same goal. Interactive educational outreach ("academic detailing") is a practical implementation of social marketing principles to improve antibiotic use. Public education programs directed at consumers can help to reduce the inappropriate patient demand that helps to drive much improper antibiotic prescribing.