Faced with a rapidly changing healthcare environment, primary care practices often have to change how they practice medicine. Yet change is difficult, and the process by which practice improvement can be understood and facilitated has not been well elucidated. Therefore, we developed a model of practice change using data from a quality improvement intervention that was successful in creating a sustainable practice improvement. A multidisciplinary team evaluated data from the Study To Enhance Prevention by Understanding Practice (STEP-UP), a randomized clinical trial conducted to improve the delivery of evidence-based preventive services in 79 northeastern Ohio practices. The team conducted comparative case-study analyses of high- and low-improvement practices to identify variables that are critical to the change process and to create a conceptual model for the change. The model depicts the critical elements for understanding and guiding practice change and emphasizes the importance of these elements' evolving interrelationships. These elements are (1) motivation of key stakeholders to achieve the target for change; (2) instrumental, personal, and interactive resources for change; (3) motivators outside the practice, including the larger healthcare environment and community; and (4) opportunities for change--that is, how key stakeholders understand the change options. Change is influenced by the complex interaction of factors inside and outside the practice. Interventions that are based on understanding the four key elements and their interrelationships can yield sustainable quality improvements in primary care practice.