BackgroundImplementing best practice in healthcare is complex. There is evidence to suggest that certain individuals, collectively termed ‘intermediaries,’ can contribute to implementation processes, but understanding exactly what happens and how intermediaries promote best practice is unclear.ObjectivesThe aim of this study was to evaluate the role of intermediaries in promoting infection prevention, and provide an explanation about what works, for whom, how, and under which conditions.MethodsRealist methodology was used as the underpinning explanatory framework for the study. From a concept mining of the existing literature, a set of hypothetical statements about the plausible range of context- mechanism - outcome propositions that postulate how intermediaries can contribute to promoting best practice were developed and evaluated.DesignCase studies were conducted consecutively to refine and test the propositions. Data included semi-structured interviews (n = 32), non-participant observations (n = 5) and documentation review. Data were analysed by open coding, content and pattern matching.SettingsCase studies were undertaken in two hospitals within the United Kingdom.ParticipantsPurposive sampling was used to identify individuals within the organizations who had professional or organizational responsibilities for infection prevention. The inclusion criteria were; employees of the chosen organisations who would consent to take part in the study, participants with infection prevention responsibilities, adults over 18 years with the capacity to consent. The exclusion criteria were; participants outside of the chosen organisation, participants under 18 years of age, and participants who lacked the capacity to consent.ResultsFour context-mechanism-outcome configurations contribute to advancing our understanding about the potential of intermediaries to promote best practice. Findings showed that the ways in which intermediaries watch over practice (their human surveillance), promoted better adherence with infection control practices. Particular styles and approaches used by intermediaries led to individual staff feeling personally supported. Distinct ways of providing performance feedback for staff together with the policy discourse promoted good habitual behaviours. Practice-based teaching heightened awareness of individuals’ own practice and made learning more real.ConclusionsFindings offer a new lens on the role of intermediaries in bridging the evidence to practice gap. As such they could be considered when reviewing or developing new interventions/programmes that use intermediaries to plug the gap between theory and practice. The findings could also be used to guide the design and development of new intermediary models in healthcare, to promote best practice and support the quality of patient care