I have always been interested in questions about how and why. I am drawn to details that contribute to a logical whole. And new questions always arise. How and why?As a newly graduated nurse, I began clinical work in a pediatric hospital department. Every report on an adolescent coming in intoxicated made me concerned. How and why? I took an academic course in prevention science of psychosocial problems among adolescents. I continued with second-circle studies to prepare for a career working explicitly with health promotion and disease prevention among children and adolescents. After some years in a psychiatric department for children and adolescents, I finished my studies and went on to work as a school nurse. Those were enjoyable times. However, new questions always arise. I returned to academia to pursue a master's degree in public health. I wanted to learn more about quality improvement and evaluation science. At the end of the first year of two, I had the opportunity to be part of a new research project that perfectly matched my interests in adolescents, health promotion, and disease prevention. This thesis is about that project.The desire to understand the complexities that impact human behavior combined with my professional and personal experiences have influenced how the studies within this thesis were designed, how data were interpreted, and what I have highlighted in the results and discussion sections.