The aim of our study on the use of health apps is to find out how users use health-related smartphone apps, which can be used to record sports activities or dietary behaviour, for example, in their everyday lives and how this changes over the course of three months. Using the dacadoo app as an example, we are investigating what motivates users to use such a tool, which elements of such an app they find practical and good and what they may even find annoying. We also investigate how the use of such an app affects health behaviour and well-being over time.