Observational studies cannot demonstrate causality. To demonstrate cause and effect requires an intervention study in which consumption of a nutrient, food or diet is altered in a controlled way and the effect on selected outcomes is measured. The study design process should include careful consideration of the hypothesis, duration, intervention, amount and mode of delivery, control and blinding, primary and secondary outcome measures (including assessment of background diet), statistical power, eligibility criteria, data‐collection methodology and ways of measuring and encouraging compliance. This chapter examines the different types of intervention study and then outlines some of the key factors to consider when planning such studies. It provides an overview of the major factors involved in the planning, conducting and reporting of intervention studies and highlights that local ethical approval, research governance procedures, and appropriate interpretations need to be followed.