This article argues for a more systematic use of theories in planning and evaluating quality-improvement interventions in clinical practice. It demonstrates how different theories can be used to generate testable hypotheses regarding factors that influence the implementation of change, and it shows how different theoretical assumptions lead to different quality-improvement strategies. The article summarizes and recommends a set of theories regarding change in health care, emphasizing the importance of understanding the theoretical assumptions and hypotheses behind these factors to design effective interventions. The complexity of changing practices in health care is highlighted, as interventions often involve multiple interacting factors at different levels (patients, professionals, team interactions, organizational context, and economic/political context). The article discusses various theories, including process theories and impact theories, which provide different perspectives on how to achieve effective change. Process theories focus on the implementation of change, while impact theories focus on the factors that influence the success or failure of interventions. The article also presents practical examples, such as improving hand hygiene and diabetes management, to illustrate how theories can be applied in real-world settings. It discusses different types of theories, including cognitive theories, educational theories, motivational theories, theories related to social interaction, communication, social learning, social networks, team effectiveness, professional development, leadership, organizational context, innovative organizations, and integrated care. The article emphasizes the importance of understanding these theories to design effective interventions that address the complex nature of health care change. It concludes that a systematic use of theories is essential for successful quality improvement in health care.This article argues for a more systematic use of theories in planning and evaluating quality-improvement interventions in clinical practice. It demonstrates how different theories can be used to generate testable hypotheses regarding factors that influence the implementation of change, and it shows how different theoretical assumptions lead to different quality-improvement strategies. The article summarizes and recommends a set of theories regarding change in health care, emphasizing the importance of understanding the theoretical assumptions and hypotheses behind these factors to design effective interventions. The complexity of changing practices in health care is highlighted, as interventions often involve multiple interacting factors at different levels (patients, professionals, team interactions, organizational context, and economic/political context). The article discusses various theories, including process theories and impact theories, which provide different perspectives on how to achieve effective change. Process theories focus on the implementation of change, while impact theories focus on the factors that influence the success or failure of interventions. The article also presents practical examples, such as improving hand hygiene and diabetes management, to illustrate how theories can be applied in real-world settings. It discusses different types of theories, including cognitive theories, educational theories, motivational theories, theories related to social interaction, communication, social learning, social networks, team effectiveness, professional development, leadership, organizational context, innovative organizations, and integrated care. The article emphasizes the importance of understanding these theories to design effective interventions that address the complex nature of health care change. It concludes that a systematic use of theories is essential for successful quality improvement in health care.