This paper describes the challenges of accurately measuring the upper tail of the income and wealth distributions in low- and middle-income countries. It reviews the seminal contributions in the literature on measuring the top of the income distribution and then more recent work, focusing on the challenges in doing so and the solutions that have been proposed. The paper focuses mostly on incomes, but it devotes a specific section to examining measurement of the top of the wealth distribution and the key differences compared with measuring income. It then identifies gaps in the literature and the implications for those undertaking research in this area. Finally, the paper proposes a research agenda to close the identified key gaps for practitioners, distinguishing between steps that can be undertaken with current datasets and those that require new data.