Ethnic minorities in the UK Parliament are numerically under represented, despite recent increases in the number of Black and Minority Ethnic (BME) Members of Parliament in the 2010 and 2015 General Elections. This under-representation is a problem for several reasons but especially because of the possibility that their interests are not adequately represented. In this thesis I ask the complex question of how, why, and when substantive representation of ethnic minorities takes place in the Parliament of the United Kingdom, using a multi-pronged approach. I draw on theories and concepts developed in studies of representation of other historically marginalised groups, including female political representation, gaining insights mainly from those writing in Europe and the USA. In the first of four empirical chapters I examine substantive representation by those Members of Parliament from BME communities, and who thus are descriptive representatives of those communities. Second, I explore substantive representation amongst those who operate as critical actors, who are not necessarily descriptive representatives of these communities. Third, I ask whether Members of Parliament respond in the same way to BME constituents. Finally, I test certain mechanisms that have been proposed as factors underlying substantive representation. I find considerable evidence for a link between descriptive and substantive representation, with BME Members of Parliament responding in ways that are different from their non-BME counterparts when critical events occur, in the way that they speak about and represent ethnic minorities in debates (Chapter 3). I also find that non-BME Members of Parliament, or their offices, are less responsive to an ethnic minority constituent, even when the question asked of the representative is of critical importance (Chapter 5). In each of these chapters I find evidence that both electoral incentives and the political party of the Member of Parliament are important. I also look at substantive representation without descriptive representation, or the potential for non-BME representatives to act for ethnic minorities. I find, in Chapter 4, that these critical actors are most likely to be in the Labour Party and represent ethnically diverse seats, as well as being most often found among BME Members of Parliament. In Chapter 6 I test certain mechanisms proposed as underpinning the relationship between descriptive and substantive representation. There, I find good evidence supporting intrinsic mechanisms; linked fate and a sense of responsibility to represent, and some evidence for electoral incentives as a mechanism. By taking this multi-pronged approach I am able to capture how the substantive representation of ethnic minorities takes place in the UK Parliament, from initial contacts between constituents and their Members of Parliament to how their interests are presented in the House of Commons. Substantive representation is, I argue, a journey, although not necessarily a linear one, which involves constituents' attitudes, how they communicate their concerns to their representatives, and how their representatives communicate them to Parliament. The approach I have taken has allowed me to understand how substantive representation happens at these different stages, and explore why and when representatives are motivated to act for ethnic minorities.