New policies should be informed by evidence regarding what has been shown to 'work' elsewhere. However, in many cases, there is limited evidence to inform decision-making. Embedding evaluations into the roll-out of new policy initiatives offers a useful means of examining whether policies achieve their intended outcomes and building the evidence base. This study examines the cluster randomised trial of the Primary School Free Breakfast Initiative in Wales. This involved recruiting 111 schools, before randomly allocating 55 to receive the intervention and 56 to act as a control group.