Although most international agreements are concluded for indefinite periods, the issues they address and parties’ preferences are constantly evolving. In some cases, parties seek to close any growing gaps between negotiators’ expectations and the changing context by updating their original agreement to its new circumstances. States have several formal tools at their disposal to do so, such as protocols, amendments, and addenda. We refer to this process as institutional adaptation. This paper seeks to explain why some agreements are adapted numerous times during their lifetime while others are not. It argues that state parties are more likely to adapt their international agreements when they acquire new information about their partners’ behavior, preferences, or the state of the environment. We focus on two key elements facilitating this process. The first consists of unexpected variation in treaty participation, and the second concerns the design features of the agreement. Relying on event history analysis and an original dataset of design features and membership of 371 multilateral environmental agreements (MEAs), we find that low levels of ratifications, high levels of accessions, highly institutionalized MEAs, and anticipatory design features are associated with more frequent institutional adaptation. These findings provide important lessons for the design of dynamic treaties.