Background: Reducing the rates of healthcare acquired infections require efforts that focus on changing professional behaviour as well as providing strategies for practice (Huis et al, 2011). The term intermediary has been used to refer to individuals “within the practice environment who can influence nurses toward specific goals” (Ferguson et al, 2004: 325). Aim: to evaluate the role of the intermediary in promoting best practice in infection prevention and control Methods: Realist evaluation (Pawson & Tilley, 1997) was used to elicit an explanation of the contexts in which, and mechanisms through which, intermediaries support infection prevention and control best practice. A realist synthesis of the literature was conducted, and mixed methods case studies were undertaken within two NHS hospitals in the United Kingdom, with data comprising of interviews, non-participant observations, and documentation review. Data were integrated and analysed to develop a refined set of findings that explained the intermediary’s role in promoting best practice. Stakeholders’ involvement throughout the study was important. Discussion: Findings showed that intermediary proximity and presence in clinical areas is important, and together with ways in which intermediaries ‘watch over’ practice, can promote better adherence to infection control practice. Intermediaries’ styles and approaches are influential, and can lead to clinical staff feeling individually supported. Policy discourse and enforcement, and the ways in which intermediaries provide feedback contribute to promoting good habitual behaviours. Conclusion: The findings of this study are informative for both policy and practice because they identify factors that could be built into future intermediary programmes. Fundamentally, organisational commitment to invest in programmes that are clinically embedded and which support the development of individuals who have the potential to influence practice is warranted.