Objectives To explore the 'added value' that general practitioners (GPs) bring to commissioning in the English NHS. We describe the experience of Clinical Commissioning Groups (CCGs) in the context of previous clinically led commissioning policy initiatives. Methods Realist evaluation. We identified the programme theories underlying the claims made about GP 'added value' in commissioning from interviews with key informants. We tested these theories against observational data from four case study sites to explore whether and how these claims were borne out in practice. Results The complexity of CCG structures means CCGs are quite different from one another with different distributions of responsibilities between the various committees. This makes it difficult to compare CCGs with one another. Greater GP involvement was important but it was not clear where and how GPs could add most value. We identified some of the mechanisms and conditions which enable CCGs to maximize the 'added value' that GPs bring to commissioning. Conclusion To maximize the value of clinical input, CCGs need to invest time and effort in preparing those involved, ensuring that they systematically gather evidence about service gaps and problems from their members, and engaging members in debate about the future shape of services.