Deterrence is an attempt to influence another actor’s assessment of its own interests. It seeks to prevent an undesired behavior by convincing the party who may be contemplating such an action that its cost will exceed any possible gain. Deterrence presupposes that decisions are made in response to some kind of rational cost-benefit calculus, that this calculus can be successfully manipulated from the outside, and that the best way to do so is to increase the cost side of the ledger. This chapter challenges all three assumptions. It argues that deterrence is a flawed strategy of conflict management, applicable only to a limited number of cases, and spells out many of the reasons why this is so. Our critique of deterrence has three interlocking components: political, psychological, and practical. Each exposes a different set of problems of deterrence in theory and practice.