A duty to vote may be many things. It may be no more than an obligation to cast one's ballot as self-interestedly or as altruistically as one pleases. It may a requirement to vote for the political party most likely to yield the highest social welfare. It may be a requirement to choose between competing parties in the interest of one's social class or with recognition of the needs of the poor. It may include a requirement to inform oneself about the issues in an election. This paper begins with a critique of the argument denying any duty to vote because, as with participants in the market, there is no conflict between selfinterest and public interest in the choice whether to vote or abstain. The core of the paper is a discussion of several interpretations of the duty to vote, and there is a brief review of pros and cons of compulsory voting.