This article defends deliberative democracy against the problem of tacit knowledge. It has been argued that deliberative democracy gives a privileged position to linguistic communication and therefore excludes tacit forms of knowledge which cannot be expressed propositionally. This article shows how the exclusion of such knowledge presents important challenges to both proceduralist and epistemic conceptions of deliberative democracy, and how it has been taken by some to favour markets over democratic institutions. After pointing to the limitations of market alternatives, deliberative (...) democracy is defended by arguing that tacit knowledge can be brought into deliberation through the mechanism of trust in testimony. By trusting the testimony of a speaker, deliberators are able to act on knowledge even without it being explicitly expressed. The article then goes on to discuss the implications of this defence for deliberative theory, and particularly, the forms of reason which deliberative democrats must see as legitimate. (shrink)