Reaction is a subject usually avoided by political theorists, since it raises awkward historical, philosophical and political questions. Perhaps philosophers of history might make better sense of it. In this article I claim that reaction has to be understood in relation to the concepts of revolution, tradition, progress and conservatism. I argue that the specific meaning of reaction is a response to the specific action that establishes the principle that order should be established only on enlightened principles. The few theorists who have dealt with reaction have disagreed about whether it is the same as conservatism or not. I show that reaction is not an element in what I call a status quo conservatism, though it is an element in any conservatism conceived more broadly. I characterise reaction in full as the attempt to reverse the establishment of the principle that only enlightened principles shall be the basis of political order, the attempt to resist the further establishment of those enlightened principles, and also the attempt to criticise contemporary enlightened politics in terms of the unenlightened standards which existed before the revolution.