Is all economic inequality bad? Or, like cholesterol, might there be better and worse forms of inequality? In this Policy Research Talk delivered on January 22, 2019, World Bank Senior Adviser Francisco Ferreira discussed how the concept of inequality of opportunity is used by economists. He briefly reviewed the normative underpinnings and philosophical roots of the theory, provided some empirical illustrations, and then presented formal economic approaches to the measurement of inequality of opportunity. Ferreira also commented on the close relationship between the concepts of inequality of opportunity and socio-economic mobility; and offered a few examples of applications in the areas of economic growth and social policy evaluation.