Referendums, like any large-scale processes of democratic decision-making, need to take place within a framework of rules. This chapter examines such rules. It surveys how referendums are regulated in Europe today, focusing on five key dimensions: when referendums take place; how the vote is structured; the status of the result; the conduct of the campaign; and how the rules are enforced. It also assesses how referendums ought to be regulated. It finds that there are many important gaps in current regulatory frameworks: much more could be done to ensure that referendums play the most positive role they could in the overall democratic system.