Child poverty can be defined and measured in several different ways. This chapter presents various definitions and indicators of poverty and child poverty, as well as the general principles of measuring child poverty, with a special focus on income-based indicators and deprivation indicators. The chapter also illustrates that the prevalence of child poverty is dependent on which poverty indicator is used. During the last decades, countless indicators have been developed to measure poverty in various forms, including child poverty. This chapter outlines the different ways of defining and measuring child poverty. The purpose is not to provide an exhaustive list of child poverty definitions and indicators, but rather to highlight some of the most prominent approaches. The material covered in the chapter serves as a basis for subsequent chapters on specific countries.