The Equalities Review is an independent panel set up by the UK government in 2005 to investigate the persistence of social inequalities and to make recommendations for the development of a unified Commission for Equality and Human Rights. This paper was originally written for the Review. It canvasses possible responses to the questions, ‘equality between whom?’ and ‘equality of what?’. It argues that equality of outcome is intuitively appealing but risks ignoring variations in need, differences in values and preferences, and the importance of individual agency. A broad interpretation of equality of opportunity, such as is provided by the capability approach, can address these limitations, by focusing on the substantive freedom enjoyed by individuals. Substantive freedom may be limited by a lack of personal resources, or by the economic, social, political, cultural, and environmental conditions context in which the individual is operating. The paper concludes by identifying, and indicating solutions to, a number of measurement issues that arise in operationalising the capability approach.