This chapter is concerned with the measurement of diversity. It distinguishes two main types of indices: fractionalization indices and disenfranchisement indices. Fractionalization indices capture the ethnolinguistic mosaic of existing societies. They allow cross-country or cross-regional comparisons and examination of differences between various economic and political systems, institutions, and outcomes influenced by the diversity of societies. The chapter also discusses polarization indices, which are based on the same fundamentals. Polarization, as well as fractionalization, entails several groups with similar or identical members whose linguistic or ethnic characteristics are substantially different from those in other groups. However, in addition to exogenous ethnolinguistic distances between groups, polarization also introduces the idea of identification and alienation. Disenfranchisement indices are related to the notion of linguistic disenfranchisement caused by government policies. In addition, the chapter discusses the links between fractionalization, disenfranchisement, and communication indices, which were introduced in Chapter 3.