The chapter describes how relative deprivation theory can be used to produce valid and reliable estimates of the extent and nature of child poverty both within and between countries. A worked example using data from Mexico is provided to show how the methodology can be used to produce scientific estimates of poverty which conform to national standards. The chapter also examines the practical and theoretical problems of adapting three commonly-used poverty measurement methods (World Bank's ‘dollar-a-day’, the Wealth/Asset Index and the recent UNDP's Multidimensional Poverty method) to study the child poverty.