Social mobility and improving the performance of disadvantaged pupils is a focus for policy in England, Northern Ireland and Wales. The findings from PISA 2018 provide a unique opportunity to explore the impact of disadvantage on pupils aged 15, to contextualise achievement outcomes with information about home-life and school, as well as providing an international perspective. In particular, PISA provides the opportunity to explore the impact of disadvantage on attainment using a more sophisticated measure of disadvantage than the binary 'eligible for free school meals or not' by measuring socioeconomic status on a continuous scale based on pupils' responses to questions about their parents' background and education, and possessions in their homes. This research brief summarises the key findings from the analysis of disadvantaged pupils in England, Wales and Northern Ireland. Comparisons are also made with three OECD countries; Estonia and Canada as high-achieving countries with greater equity, and the Republic of Ireland to complement the simultaneous analysis PISA 2018 additional analyses: Learning from other countries (Sizmur et al., forthcoming). [For the full report, see ED612752.]