The idea that ignorance can be rational may at first seem paradoxical. This chapter consider what it means to be rationally ignorant, contrasting rational ignorance with irrational or inadvertent ignorance. It explains why such ignorance is both widespread and often desirable. The chapter strikes a cautionary note, focusing on situations where individually rational ignorance could lead to harmful results for society as a whole. By far the most significant such case is the rational ignorance of voters about politics. Because effective democratic government requires an informed electorate, the rational ignorance of voters could be a serious problem for democracy. A different rebuttal to the rational ignorance critique of democracy is advanced by advocates of the “miracle of aggregation,” which holds that the electorate collectively makes better decisions than individual voters can. The extent to which rational ignorance undermines democracy depends in part on one’s normative theory of democratic participation.