Many multilateral environmental treaties have established committees that monitor compliance and/or facilitate implementation. These committees can be triggered in a number of ways, e.g., when a State party seizes the committee concerning another State party’s compliance or implementation. This type of trigger resembles the most traditional judicial proceedings, as it opposes two States, and has the potential to lead to decisions of non-compliance. It is perceived to be more confrontational than facilitative. State-to-State triggers therefore sit in between judicial and non-judicial procedures, and between facilitation on the one hand and enforcement on the other. State-to-State triggers have only been used a handful of times. This chapter explores why such triggers have been sparsely used. It first explains how State-to-State triggers were established and describes the instacnes in which they have been used. It then identifies and discusses two main challenges faced by State-to-State triggers: challenges related to the perception and behaviour of States vis-à-vis such triggers, and challenges related to institutional design and procedural mechanisms of State-to-State procedures.