Control theory is about the processes underlying the behaviour of self-regulating agents. It proposes that behaviour is regulated by a negative feedback loop, in which the agent compares the perception of its current state against a goal state and will strive to reduce perceived discrepancies by modifying its behaviour. Although studies in health informatics often do not report the use of this theory, the principle of a negative feedback loop underlies many applications in the field. This chapter describes how control theory fits within health informatics, discussing its role in the development and assessment of audit and feedback interventions in healthcare. Control theory has been used to synthesise evidence of audit and feedback, and to design and evaluate interventions to improve the quality of blood transfusion practice, cardiac rehabilitation, and intensive care. This has driven progress in our understanding of the underlying mechanisms of audit and feedback for improving health care, and has helped to design better interventions.