Recent national and international evidence suggests that the wellbeing of young people in the UK has decreased. The Programme for International Student Assessment (PISA) 2018 national reports for England, Wales and Northern Ireland (Sizmur et al., 2019a; Sizmur et al., 2019b; Sizmur et al., 2019c) identified cause for concern for 15-year-old pupils. Pupils' wellbeing in England, Wales and Northern Ireland was significantly lower than wellbeing in the other OECD countries on average. The comparison with PISA 2015 was also worrying; the proportion of 15-year-olds in the UK that reported being satisfied with their lives dropped at a faster rate than anywhere else, a 13 percentage point drop since 2015 (OECD, 2019). Based on PISA 2015 and 2018, this research brief summarises the key findings from the analysis of pupils' wellbeing data in England, Wales and Northern Ireland, as well as three OECD comparator countries, France, Finland and Korea. [For the full report, see ED612544.]