The aim of this feasibility study was to outline how future research might be designed to estimate the scale and nature of NMW underpayment in the informal economy. This was a hard-to-measure area, presenting a number of challenges. We focused on two broad methodological themes – namely the methodological challenges and the data challenges – in structuring out approach. We over-layed this with a general challenge about measuring the informal economy: the behaviours around non-compliance necessarily mean data was riddled with systematic biases for under-reporting. Our approach also included expert reviews, recognising such input was needed to assure the approaches recommended.