Background: A core component of the Health and Social Care Act 2012 (Great Britain. Health and Social Care Act 2012. London: HMSO; 2012) was the idea of devolving to general practitioners (GPs) a health service leadership role for service redesign. For this purpose, new Clinical Commissioning Groups (CCGs) were formed in the English NHS. Objectives: This research examined the extent to which, and the methods by which, clinicians stepped forward to take up a leadership role in service redesign using CCGs as a platform. Design: The project proceeded in five phases: (1) a scoping study across 15 CCGs, (2) the design and administration of a national survey of all members of CCG governing bodies in 2014, (3) six main in-depth case studies, (4) a second national survey of governing body members in 2016, which allowed longitudinal comparisons, and (5) international comparisons. Participants: In addition to GPs serving in clinical lead roles for CCGs, the research included insights from accountable officers and other managers and perspectives from secondary care and other provider organisations (local authority councillors and staff, patients and the public, and other relevant bodies). Results: Instances of the exercise of clinical leadership utilising the mechanism of the CCGs were strikingly varied. Some CCG teams had made little of the opportunity. However, we found other examples of clinicians stepping forward to bring about meaningful improvements in services. The most notable cases involved the design of integrated care for frail elderly patients and others with long-term conditions. The leadership of these service redesigns required cross-boundary working with primary care, secondary care, community care and social work. The processes enabling such breakthroughs required interlocking processes of leadership across three arenas: (1) strategy-level work at CCG board level, (2) mid-range operational planning and negotiation at programme board level and (3) the arena of practical implementation leadership at the point of delivery. The arena of the CCG board provided the legitimacy for strategic change; the programme boards worked through the competing logics of markets, hierarchy and networks; and the practice arena allowed the exercise of clinical leadership in practical problem-solving, detailed learning and routinisation of new ways of working at a common-sense everyday level. Limitations: Although the research was conducted over a 3-year period, it could be argued that a much longer period is required for CCGs to mature and realise their potential. Conclusions: Despite the variation in practice, we found significant examples of clinical leaders forging new modes of service design and delivery. A great deal of the service redesign effort was directed at compensating for the fragmented nature of the NHS – part of which had been created by the 2012 reforms. This is the first study to reveal details of such work in a systematic way. Future work: Further focus is needed on the emergent sustainability and transformation plans and the locality/hubs/primary care homes that integrate care across population groups. Funding: The National Institute for Health Research Health Services and Delivery Research programme.