Using real-life data to better understand peoples behavior in their natural environments Reallife and realtime data collection methods offer unprecedented opportunities to advance the understanding and promotion of physical and mental health. Everyday assessments offer valuable insights into the behavior of individuals in their natural environment, providing a more complete picture than experimental studies conducted in the laboratory. These insights can also be used to design more effective interventions to promote physical and mental health, for example, by tailoring intervention content to individual states using predictive algorithms. This volume shows that person-level moderators and contextual factors are crucial to consider when studying everyday life behaviors in the context of health and disease. Contributions include studies that primarily use digitally supported, multidimensional assessments in everyday life over a period of up to four weeks. Self-report and objective measures, such as current social media use or different biological parameters, are also used to address research questions in clinical and health psychology.