Popularising summary: Exploring the mental health gap between adolescents with and without a migration background - The role of policies Migration has been an important part of human history and still plays an important role today. In 2020, it was estimated that there were around 281 million international migrants, with many of them being under the age of eighteen years old. In the future, the number of migrants and underage migrants is only expected to rise even further. This is because of poverty, conflicts, disasters, damage to the environment and a lack of security and access to basic services. This predicted increase highlights migrant adolescent mental health as an important public health issue and research topic. Although a large amount of research has already been done, the research field of migrant adolescent mental health has produced contradictory findings. While some studies found that migrant adolescents have worse mental health than native adolescents, others found the opposite. Furthermore, even within the group of migrant adolescents, differences have been found based on their migration background. The presence of differences in the mental health gap between adolescents with and without a migration background across countries could suggest that these mixed findings may be due to differences in policies. Therefore, our research looked at whether differences in health and migration policies could explain these differences in the mental health gap. By doing a multilevel analysis with psychosomatic complaints and life dissatisfaction as outcome variables, no evidence was found that health and migration policies predict the cross-national differences in the mental health gap. One exception found that health policies have a negative effect on the psychosomatic complaints of first-generation EU migrant adolescents, compared to no effect of health policies in native adolescents. However, this effect is probably the result of the small number of adolescents in this group. It is really important to study migration because migrants are among the most vulnerable groups in society. They experience poor living, working and housing conditions, physical and mental health problems and discrimination, social exclusion and violence. Although a lot of research has already been conducted, it is important to continue researching this topic since the nature and impact of migration have been changing over time as a result of globalisation and geopolitical, environmental and technological changes. Furthermore, mental health problems are the leading cause of reduced human productivity and poor social functioning. Additionally, an association has been found between mental health problems and physical health problems. Moreover, researching the adolescent migration population is especially important because adolescence is a critical period for the development of identity, social relationships and emotion regulation. Problems during this stage can have long-term effects on their mental health, integration, and academic and working life. In addition, improving the mental health of migrants could also benefit the host population by improving the integration process and boosting social and economic development. Furthermore, “The Edinburg Declaration on Migration, Ethnicity, Race and Health” and the “Global Research Agenda on Health, Migration, and Displacement” of the World Health Organization emphasise putting migrant health on the research agenda. Lastly, studying the mental health of migrant adolescents aligns with the principles of universal health coverage, health as a human right, and the Sustainable Development Goals. Although we did not find the results we hoped for, we got closer to a better understanding of migrant adolescent mental health and the existing contradiction in literature. This dissertation could be an inspiration for further research which should focus on using more diverse mental health outcomes since their relation with policies could be different. Additionally, research should try to solve the policy-practice gap. When implementing policies, things can go wrong leading to not getting the results that were hoped for. Identifying and researching what really affects the mental health of migrant adolescents could be very insightful. Moreover, although it is more likely that the one statistically significant cross-interaction effect is due to the small sample size, research could try to identify unique characteristics or processes of the group of first-generation EU migrant adolescents that could explain these results.