n order to assess the different social, economic, health and wellbeing outcomes across different ethnic minority groups living in Britain, we need to collect and analyse data based on measures of identity, which can expose unequal experiences and the effects of discrimination. As discussed in the introductory chapter, these identity categories are not straightforward and reflect political choices made in particular historical contexts. The categories used by the state on which these data are based have shifted over time, and people’s identification with them will also change. This chapter will introduce the nature of ethnic diversity in the UK, giving an overview of the size and location of ethnic groups in the UK and how they have changed over time. It will consider the ways in which Britain’s history as a global empire and related migration have shaped the categories we use today, which, in turn, determined the nature of ethnic diversity in the UK. The chapter will examine historic migration flows and current ethnic groups in the UK, the age structures of different ethnic groups, which reflect patterns and periods of migration as well as fertility and mortality patterns. Finally, it will consider the ways in which processes of migration have produced distinct residential patterns for different ethnic groups and how these are changing.