Addressing complex public health problems, such as smoking, obesity and mental health, requires complex, often multilevel, interventions. Given the costs associated with delivering such interventions and the possibility of unanticipated harm, they need to be evaluated using the most robust methods available. It is important, where possible, that public health interventions and their proposed evaluation designs are optimised prior to being subject to an expensive evaluation of their effectiveness, through rigorous assessment of their feasibility.