This chapter discusses the Gini index and Gini mean difference, two closely related statistics that have vastly different applications. These applications include traditional ones such as income and wealth as well as nontraditional ones arising in such diverse fields such as digital imaging, genetics, and astronomy. The Gini index and Gini mean difference are defined in terms of the Lorenz curve. The Lorenz curve allows the development of a geometric intuition regarding the quantities involved. The curve provides an interesting perspective on how the Gini index and the Gini mean difference change under various