Increasingly, political pressure on governments to act to limit perceived abuses of tax systems is leading to hastily introduced, ill-targeted legislation of dubious quality. In extreme cases, such legislation can even be retroactive. This approach sits uneasily with the rule of law, that is, the principle that a citizen of a country should be able to identify and understand the law as it applies to him or her on any given day. To what extent can departures from the rule of law be justified by the need to preserve the integrity of tax systems? What can be done where they are not? Can overarching principles of human rights law be invoked to constrain abuses of power by governments?