Objective: Refine the programme theory for OptiBreech Care Design: Concurrent mixed methods implementation process evaluation Setting: 6 NHS hospitals in England participating in the OptiBreech 1 Feasibility Study Sample: 15 women planning a vaginal breech birth at term and 6 breech lead midwives Methods: Outcomes were recorded on case report forms and descriptively analysed. Interviews were recorded, transcribed and analysed using the Theoretical Framework of Acceptability. Iterative analysis informed subsequent interviews and the on-going process of implementation across sites. Main Outcome Measures: Acceptability of service delivery models and their outcomes. Results: Actively recruiting Trusts implemented services through a dedicated clinic and/or a proficient intrapartum support service, organised and provided primarily by a Breech Specialist Midwife. While we identified challenges, this model has achieved 93% fidelity to the intervention’s goal of ensuring attendance of OptiBreech-trained professionals at vaginal breech births, and it is highly acceptable to women. Our initial suggested model of a multi-disciplinary team composed of 5 obstetricians and 5 midwives does not appear feasible, due to very low overall current breech experience levels and the context of current pressures on NHS services. Conclusions: Appointment of a Breech Specialist Midwife, whose role is to co-ordinate a dedicated clinic, training and a proficient intrapartum care team, appears to be highly acceptable to women. This model appears to be a feasible implementation strategy, in order to test the safety and effectiveness of OptiBreech Care in a clinical trial, but further work needs to be done to develop sustainability.