This thesis is a report of a theory driven study designed to evaluate the role of the 'intermediary' in promoting best practice in infection prevention and control. Study aim and objectives: The aim of the study was to seek the programme theories that show how intermediaries might promote best practice in infection prevention and control; to determine what works, for whom, how, and in what respects. Methods: Realist evaluation was used to elicit a better understanding of the mechanisms and contexts that lead to outcomes (CMOs) for the role of intermediaries in infection prevention and control programmes. A realist review led to case studies being conducted consecutively within two NHS hospitals in the United Kingdom, with data comprising of interviews (n=32), non-participant observations (n=5) and documentation review. Findings: Four conjectured CMOs were developed and refined over two case studies. The study has shown that intermediary presence and proximity in clinical areas is important, and together with ways in which intermediaries watch over practice (through overt and covert visibility), can promote better adherence to infection control practice. The theory of self-surveillance emerged from the findings to show how intermediaries can promote self- monitoring by clinical staff. Intermediaries' styles and approaches are important, and can lead to clinical staff feeling individually supported. Policy discourse and enforcement, and the ways in which intermediaries provide feedback can contribute to promoting good habitual behaviours. Practice based educational approaches are more likely to focus individuals' attention on infection control and heighten awareness of infection control in clinical areas.