To design, implement and monitor effective child well-being policies, policy-makers need data that better capture children’s lives, measure what is important to them and detect emerging problems and vulnerabilities early on. Despite improvements in recent decades, there are still important gaps in both national and cross-national child data. Countries can achieve progress if the right actions are taken. Measuring What Matters for Child Well-being and Policies lays the groundwork for improved child well-being measurement and better data to inform better child well-being policies. It outlines an “aspirational” framework for child well-being measurement, setting out which aspects of children’s lives should be measured, and how, to better monitor child well-being. It also outlines priorities for child data development and identifies key data gaps, all with the aim of motivating improvements in child data infrastructures.