Background: Pelvic Floor Muscle Training (PFMT) has been shown to be effective for pelvic organ prolapse in women, but its implementation in routine practice is challenging due to lack of adequate specialist staff. It is important to know if PFMT can be delivered by different staff skill mixes, what barriers and facilitators operate in different contexts, what strategies enable successful implementation and what are the underlying mechanisms of their action. PROPEL intervention was designed to maximise the delivery of effective PFMT in the UK NHS using different staff skill mixes. We conducted a realist evaluation (RE) of this implementation to understand what works, for whom, in what circumstances and why. Methods: Informed by the Realist and RE-AIM frameworks, the study used a longitudinal, qualitative, multiple case study design. The study took place in five, purposively selected, diverse NHS sites across the UK and proceeded in three phases to identify, test and refine a theory of change. Data collection took place at 4 time points over an 18 month implementation period using focus groups and semi-structured interviews with a range of stakeholders including service leads/managers, senior practitioners, newly trained staff and women receiving care in the new service models. Data were analysed using thematic framework approach adapted to identify Context, Mechanism and Outcome (CMO) configurations of the RE. Results: A heightened awareness of the service need among staff and management was a mechanism for change, particularly in areas where there was a shortage of skilled staff. In contrast, the most established specialist physiotherapist-delivered PFMT service activated feelings of role protection and compromised quality, which restricted the reach of PFMT through alternative models. Staff with some level of prior knowledge in women's health and adequate organisational support were more comfortable and confident in new role. Implementation was seamless when PFMT delivery was incorporated in newly trained staff's role and core work. Conclusion: Roll-out of PFMT delivery through different staff skill mixes is possible when it is undertaken by clinicians with an interest in women's health, and carefully implemented ensuring adequate levels of training and ongoing support from specialists, multi-disciplinary teams and management.