ObjectiveThe reform in the English National Health Services (NHS) under the Health and Social Care Act 2012 is unlike previous NHS reorganisations. The establishment of clinical commissioning groups (CCGs) was intended to be ‘bottom up’ with no central blueprint. This paper sets out to offer evidence about how this process has played out in practice and examines the implications of the complexity and variation which emerged.DesignDetailed case studies in CCGs across England, using interviews, observation and documentary analysis. Using realist framework, we unpacked the complexity of CCG structures.Setting/participantsIn phase 1 of the study (January 2011 to September 2012), we conducted 96 interviews, 439 h of observation in a wide variety of meetings, 2 online surveys and 38 follow-up telephone interviews. In phase 2 (April 2013 to March 2015), we conducted 42 interviews with general practitioners (GPs) and managers and observation of 48 different types of meetings.ResultsOur study has highlighted the complexity inherent in CCGs, arising out of the relatively permissive environment in which they developed. Not only are they very different from one another in size, but also in structure, functions between different bodies and the roles played by GPs.ConclusionsThe complexity and lack of uniformity of CCGs is important as it makes it difficult for those who must engage with CCGs to know who to approach at what level. This is of increasing importance as CCGs are moving towards greater integration across health and social care. Our study also suggests that there is little consensus as to what being a ‘membership’ organisation means and how it should operate. The lack of uniformity in CCG structure and lack of clarity over the meaning of ‘membership’ raises questions over accountability, which becomes of greater importance as CCG is taking over responsibility for primary care co-commissioning.