Since 1948, the National Health Service (NHS) has undergone repeated organisational changes,1 but none so large or complex as the 2012 Health and Social Care Act (HSCA). Its stated purpose was to liberate the NHS from direct ministerial control, transferring responsibility to the independent body, NHS England, to deliver on an annual mandate from ministers. NHS England oversees local Clinical Commissioning Groups (CCGs), which were intended to allow general practitioners to decide on the commissioning of certain services from ‘any qualified provider’. An extensive network of new entities was created to facilitate this, such as Clinical Senates and Health and Wellbeing Boards, while some existing bodies had their powers enhanced, such as the economic regulator Monitor. The resulting NHS system is extremely complex, with many uncertainties about how it should work and examples of a reality which differs from stated intentions. For example, the newly ‘liberated’ head of NHS England must now meet with the Secretary of State weekly, so that direct ministerial control continues. Whenever large-scale reforms take place, they should be accompanied by robust evaluation to inform future policy. The latest change to the English NHS has been described as ‘so large you could probably see it from space’,2 but despite this we may never know its true impact on population health. Previous NHS reforms illustrate the challenges involved in the HSCA’s evaluation.3,4 A first challenge is political. Politicians resisted proposals to evaluate the introduction of the ‘internal market’ in the 1990s, so its impacts remain debated today. The second challenge is operational. New Labour committed funding to policy evaluations that improved understanding of processes by which change occurred, but failed to incorporate evaluation into actual policy design and implementation, so limiting the validity of their conclusions. This was again the case with the HSCA, as piloting CCGs or other elements of the reform was excluded, so precluding randomised designs which could permit causal inferences. In 2013, the Department of Health (DH) called for research to evaluate the HSCA’s impact, issuing a second call when DH found submitted bids left ‘significant gaps’ and would not allow ‘an assessment of impact across the whole suite of reforms’.5 Both calls asked for evidence of improved outcomes, with the first call stating: ‘All projects are expected to propose an appropriate explanatory framework or counterfactual, where this is feasible, against which the effects of the reforms can be evaluated and attributed’.6 In this paper we ask, how would we know if the NHS reforms are working or not? We evaluate characteristics of the HSCA that fulfil ‘natural policy experiment’ criteria and propose alternative feasible designs.