This thesis offers an account of the justification of democracy and its proper relationship with the market. It sets out a justification of democracy that I call impartial instrumentalism. According to this account, a political system is justified by its tendency to make good decisions. However, this should be assessed in a way that is impartial between substantive conceptions of justice. In practice, impartial instrumentalism recommends those political systems that promote the knowledge and moral motivation of decision-makers. Democracy is justified as the best political system according to this standard. My account contributes to debates about epistemic democracy by integrating considerations of motivation, and by clarifying the relationship between instrumental and epistemic justifications of democracy. The second half of the thesis evaluates the relationship between markets and democracy in light of this account. I begin by asking whether democracies should be restricted in their ability to intervene in the market. I evaluate the most plausible contemporary argument for this, that of Ilya Somin. Somin’s argument does not meet the test of impartial instrumentalism, because he underrates the need for collective decisions and is not impartial between different conceptions of morality. Next, I ask how markets can influence democracy, and how we should respond to these influences. I examine two case studies: the funding of political speech, and capital strikes (situations in which governments retreat from policies out of fear of how markets will react). In both cases, markets can impede the proper working of democracy. For the former case, I propose a policy solution. However, the latter case raises unavoidable trade-offs between democracy and the economic benefits of trade. In this thesis, I defend a normative account of constitutional design. On the basis of this account, I argue that democracy should generally have priority over the market.