In the practical world of policymaking and implementation, knowledge about a treaty partner's compliance with the terms of an agreement is essential. Without information about the behaviour of the other party, each side has good reasons to defect from the agreement; anticipating this dynamic, both parties may fail to reach an agreement in the first place. This chapter presents a survey of formal monitoring provisions in a random sample of international agreements which covers and is conditional on four issue areas: economics, environment, human rights, and security. It shows that monitoring provisions are indeed an important feature of international agreements: almost six out of ten agreements incorporate some form of explicit monitoring provision. In the coincidence of uncertainty about behaviour and incentives to defect, states are willing to cede sovereignty by involving external entities, such as intergovernmental organizations, in the monitoring process. International organizations may be especially important information providers in cases where private actors are either less reliable or less able to reveal non-compliance.