Unhealthy lifestyle behaviours are considered modifiable risk factors for many diseases. Lifestyle interventions that target these behaviours need rigorous evaluation to assess their effectiveness. The randomized controlled trial is the study design of choice when it comes to the evaluation of interventions. However, lifestyle interventions are often complex and subject to several important issues, such as patient preference and non-adherence, that may threaten the internal and external validity of studies. There is a strong demand for high-quality randomized controlled trials of interventions that promote healthy lifestyle behaviours. With this tutorial we aim to provide guidance in the choice of an optimal randomized controlled trial design in future trials of lifestyle interventions.