In this case study, we summarize a large-scale randomized controlled trial conducted by Public Health England Behavioural Insights. The randomized controlled trial compared which of two invitation letters was most likely to increase uptake of the National Health Service Diabetes Prevention Programme among patients at high risk of developing diabetes. These patients received an invitation letter to attend the National Health Service Diabetes Prevention Programme, as a means to introduce lifestyle changes which reduce their likelihood of developing diabetes in the future. The aim of this research project was to design the invitation letter using behavioral insights and test its effectiveness for (a) uptake and (b) adherence to the National Health Service Diabetes Prevention Programme. This case study explains in detail how the research was undertaken and introduces behavioral science frameworks that facilitate the design, testing, and ultimately implementation of behavioral interventions in the context of real-world preventive health care.