Recent national and international evidence suggests that the wellbeing of young people in the UK has decreased. In 2018, the Programme for International Student Assessment (PISA) asked participating pupils a number of questions about their wellbeing, and some of the measures can be tracked over time. Based on PISA 2015 and 2018, this report focuses results from pupils' wellbeing data in England, Wales and Northern Ireland, as well as three OECD comparator countries, France, Finland and Korea. The authors investigated changes in wellbeing as assessed by a single measure of life-satisfaction, as well as wellbeing from a wider perspective which was based on a range of questions. Therefore, two measures are referred to: 'life-satisfaction' and 'wellbeing factor'. Throughout this report, wellbeing in young people seems to be first and foremost linked to personal-, then parental- and lastly teacher-relationships. As such, the report highlights several policy recommendations that might support wellbeing in young people. [For the research brief, see ED612548.]