The adoption of the randomised controlled trial within public health research to test behavioural and lifestyle interventions means that trial researchers are increasingly expected to balance ambitions of methodological rigour and social relevance in the performance of a trial. Striking this balance is particularly important when it comes to the issue of participant compliance. This article draws from fieldwork among researchers in an exercise trial in Denmark to explore the work entailed in achieving and measuring participant compliance from a distance. By drawing on perspectives from surveillance studies, we focus on the practices, technologies and forms of knowing involved in aligning participant’s bodies and practices with specific data production ambitions. The analysis highlights the work and challenges entailed in retaining participants within the scope of the researchers’ monitoring. In conclusion, we suggest that the firm commitment to produce quantitative data on compliance elides the work, challenges and collaborative practices entailed in achieving compliance and highlighting the challenges of ensuring compliance. The article describes the randomised controlled trial, not as a powerful governmental socio-technical apparatus, but as a fragile, situated, and fundamentally challenged surveillance system. This has important implications for ambitions to consider the complexity of behavioural interventions.