Complex interventions are commonly used in the health and social care services, public health practice, and other areas of social and economic policy that have consequences for health. Such interventions are delivered and evaluated at different levels, from individual to societal. Examples include a new surgical procedure, the redesign of a health care programme, and a change in welfare policy. The UK Medical Research Council (MRC) published a framework for researchers and research funders on developing and evaluating complex interventions in 2000 and revised guidance in 2006.[1] Although these documents continue to be widely used and are now accompanied by a range of more detailed guidance on specific aspects of the research process,[2–5] there have been important conceptual, methodological and theoretical developments since 2006. These developments have been addressed in a new framework commissioned by the National Institute of Health Research (NIHR) and the MRC.[6] The framework aims to help researchers work with other stakeholders to identify the key questions about complex interventions, and to design and conduct research with a diversity of perspectives and appropriate choice of methods.