Internet interventions for common mental disorders, such as depression and anxiety disorder, have become popular in research and are now gradually implemented in practice. However, many interventions suffer from low adherence of users which in turn leads to reduced effectiveness. In this context, research on adherence has frequently outlined the need for a better understanding of user engagement. To this end, our goal is to adapt, translate and validate a questionnaire for user engagement and use this questionnaire in several randomised controlled trials. From the results, we hope to get insights into how user engagement affects adherence and therapy success.