Democracy has been encountering an increasing number of critics. Whether it comes from a sympathy for autocrats, free-markets, or the more knowledgeable, this increasing democratic scepticism often takes an epistemic form. Democracy's critics argue that democratic procedures and institutions are unlikely to make good decisions or produce good outcomes in terms of justice or the common good, and should, therefore, be restricted if not completely rejected in favour of its more able alternatives. In the face of such scepticism, this thesis develops an epistemic theory of deliberative democracy. This theory has two principal aims. The first is to analyse and define the epistemic properties of deliberative democracy, and the second is to clarify the possible role epistemic values can play in a wider justification of democratic rule. In accordance with the first, the thesis analyses the ability of deliberative democratic institutions to make good or correct decisions in comparison to a broad range of prominent alternatives. These include traditional rivals such as autocracy and aristocracy, but also more modern and less considered alternatives such as free-markets, limited epistocracy and forms of technical calculation. Through these comparisons, it is argued that we have no good or clear epistemic reason to reject democracy. Deliberative democracy is found to be epistemically superior to many of its alternatives and epistemically equivalent to even its best competitors. The thesis, therefore, mounts a strong reply to democracy's epistemic sceptics. The analysis, however, also helps clarify which form of deliberative democracy is epistemically most valuable, pointing to the value systems approaches which give a prominent role to direct citizen deliberation. The epistemic theory of deliberative democracy also aims to clarify what role epistemic values can play in a wider justification of democratic rule. The thesis argues that deliberative democracy is epistemically superior to many of its rivals and no worse epistemically than even its best alternatives. This suggests that although epistemic values cannot mount a stand-alone defence of democracy, democrats would only be required to defend very weak non-epistemic values to produce a mixed justification. Far from being 'rule by the incompetent many' and therefore highly reliant on procedural values, the thesis will demonstrate that epistemic values can carry significant weight in an argument for democratic rule.