The purpose of this chapter is to help provide understanding of how inequality is measured, what kinds of choices are made in making these measurements, and the shortcomings and strengths of these different types of measurements resulting from these choices. This chapter discusses the practical, technical, ethical, and political issues surrounding the measuring of economic inequality in its various forms. First, it describes where the data used to measure inequality comes from and why different sources of data, such as tax or survey data, might result in meaningfully different measurement outcomes. In addition, data might measure inequality of income, consumption, or wealth. Next, the various commonly used “axioms,” or general principles, applied to measures of monetary inequality, such as scale and population invariance and anonymity, are defined and discussed. Some of the most common measures of monetary inequality, such as the Gini coefficient and Lorenz curve, the generalized entropy measurements such as the Atkinson and Thiel Indices, and measures based on shares and proportions such as the Palma ratio, are also explained. Next, the article explores the differences between horizontal and vertical inequality, and between relative and absolute inequality, along with some commonly used measures of each. When measuring global inequality, a choice in focus between interpersonal and inter-country inequality would also necessitate different measurement choices. Finally, the article provides a summary of various debates about how what to measure and how to measure it can meaningfully impact empirical results and, in turn, can have “real-world” effects on perceptions, policy priorities, and ultimately economic outcomes. Who gets to choose how inequality is defined and measured, and why they make choices that they make, are thus also important issues of study.