With the growing body of research on the working poor, a broader discussion of concepts and measurement has evolved. In-work poverty is a hybrid concept which – in contrast to concepts such as low wage work – combines the labour market position of an individual with a poverty perspective focusing on the living standard at the household level. Defining and measuring in-work poverty requires answers to two questions: ‘Who is a worker?’ and ‘What is poverty?’. The chapter discusses both and provides an in-depth discussion of measurement approaches, in particular those developed by the US Bureau of Labor Statistics (BLS), Eurostat and the International Labour Organization (ILO). Furthermore, it shows empirically how the incidence and structure (for example, by gender) of in-work poverty differ according to measurement approach.