['Serious shortcomings remain in clinical care in the United States despite widespread use of improvement strategies for enhancing clinical performance based on knowledge transfer approaches. Recent calls to transform primary care practice to a patient-centered medical home present even greater challenges and require more effective approaches.', 'Our research team conducted a series of National Institutes of Health funded descriptive and intervention projects to understand organizational change in primary care practice settings, emphasizing a complexity science perspective. The result was a developmental research effort that enabled the identification of critical lessons relevant to enabling practice change.', 'A summary of findings from a 15-year program of research highlights the limitations of viewing primary care practices in the mechanistic terms that underlie current or traditional approaches to quality improvement. A theoretical perspective that views primary care practices as dynamic complex adaptive systems with "agents" who have the capacity to learn, and the freedom to act in unpredictable ways provides a better framework for grounding quality improvement strategies. This framework strongly emphasizes that quality improvement interventions should not only use a complexity systems perspective, but also there is a need for continual reflection, careful tailoring of interventions, and ongoing attention to the quality of interactions among agents in the practice.'] Is primary care practice transformation hard work : insights from a 15-year developmental program of research?