While physicians are directly responsible for only a small proportion of healthcare expenditures, their decisions influence the majority of medical costs. Because practice variations among physicians have been demonstrated abundantly, strategies to modify practice patterns have evolved. This article addresses the different factors that influence variations in patterns of practice, including physician characteristics, knowledge, beliefs, and attitudes. In designing programs to modify practice patterns, educational efforts, along with practice guidelines and feedback, all have been used. Although there are mixed results for each, the use of opinion leaders and the timing of interventions proximal to practice decisions are important. The costs and benefits of interventions are discussed as a method for assessing the utility of different programs to modify practice patterns.