This chapter examines countermeasures in the law of state responsibility and thus the issue how and by whom the consequences of an internationally wrongful act are enforced. It analyses the incentive structures regarding the decision to have recourse to countermeasures by injured states (first-party countermeasures) and non-directly injured third states (third-party or collective countermeasures). The chapter first explains the role of countermeasures for the implementation of the remedies of cessation and reparation and an enforcement tool to induce compliance. It then examines the conditions of countermeasures and provides, firstly, an economic rationale for the exclusion of countermeasures when they would affect community interest obligations. Secondly, it finds that incentives for taking countermeasures vary significantly depending on the characteristics of the underlying scenario (that is, reciprocal countermeasures, retaliatory in-kind or non-in-kind countermeasures, multiple injured states, third-party countermeasures). The chapter also analyses the financial consequences of countermeasures and concludes that the compensation requirement deriving from the ARSIWA does not set the right incentives for material loss inflicted upon the target state, as the author of the prior wrongful act is the cheapest cost avoider, but should be upheld and further refined for losses incurred by innocent third states.